user7216: I have many Arduino Mini Pro, every from different seller.
Few are 3.3v and most 5v.
I had to clean table for Xmas and now I do not have any idea how to identify 3.3v Arduinos.
They do not have any marks. I bought them on ebay.
I know 3.3v has 8mhz clock but only one my arduino has big crystal with 16.000-30.

user23751: On my board the regulator is marked L0UA which is the code for a 5V regulator type LP2985, L0UB is also 5V. 
A 3.3V regulator would be marked L0RA or L0RB.

user73027: Some clones on ebay labeled "the simple" come with 4B2X. They are 3.3v but the maximum voltage is 10v instead of 12v so beware to not burn it out.

user3879: The only difference would be the regulator and crystal (8MHz on 3.3V). As @IgnacioVazquez-Abrams suggest supply power to the "raw" and measure Vcc.

user10350: Check the label of the voltage regulator, KB50 for 5V and KB33 for 3.3


user368: Plug it in and measure the voltage on pin 4 of the MCU.

user15845: The safe way (i tested it).
First connect to 3.3V USB-TTL output first (also 5V Mini can work with 3.3V).
Now prepare this sketch:
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(10000);
  digitalWrite(13, LOW);
  delay(10000);
}

Choose "Arduino Pro/Mini" with 5v/16MHz board and upload sketch.
If you see that the blinking is correct (10 sec ON/10sec OFF) - you have 5V/16MHz Arduino. Otherwise it will blink at 5 sec intervals instead of 10 sec.

user6980: The regulator should be marked K850(5.0V) or K833(3.3V).
A 5 volt part has a 16MHz resonator may be marked with "A1" or "A'N"
A 3.3 volt part has a 8MHz resonator may be marked with "80'0"
As others have indicated, you can apply up to 12V at the RAW pin, and measure the output of the regulator.

user25808: Consideration (go down for the actual solution)
If you have a lot of different China-Arduino-clones, the easiest way will be to just test it. The cloned Arduinos use a lot of different voltage regulators and often you won't even be able to find datasheets for them on Google.
Someone here provided an Arduino sketch which helps you identify the frequency with which the Atmel is running and the comments say "it is the safest method". But why? It is complicated (you need to connect the USB-TTL-converter to each of the different Arduinos and sometimes they have different pinouts, so it's error-prone - you could easily fry your Arduino if you connect the wires to the wrong pins)...

Solution
I think the safest way is just connecting a voltage in the range of 6V-12V to the RAW-pin and measuring the (regulated) output of the VCC-pin. 
There is no reason why this would kill your Arduino (in case your Arduino isn't broken - which would be good to know anyway; and of course you shouldn't short anything).
I've done a short sketch of how you should connect your Arduino Pro Mini:


RAW <---> 6V-12V
VCC <---> multimeter (5V = 5V Arduino, 3.3V = 3.3V Arduino)
One GND pins to the GND of the power source and one GND pins to the GND of the multimeter (you can take any of the Arduino pins labeled with GND)

Don't worry if your Arduino looks a little bit different, just look for the pin labels. For instance, one of my 5V Arduino Pro Minis looks like this, but also has RAW-, VCC- and GND-labels (just one of the GND pins is at a different position, make sure you check your Arduino pin-labels):


Alternative
You don't want to use the method above for some reasons? Then you need sharp eyes and some luck that the following list includes you components (some of these are collected from the other posts, some of them from my Arduinos):

3.3V:


Voltage regulators: KB33, S20K, F34V, L0RA, L0RB
Oscillators: 80e., 80'0

5V:


Voltage regulators: KB50, L05, L0UA, L0UB
Oscillators: R160JAC6s, 16.000-30, A1, A'N, A'a


(no limit or warranty for these details...)

More Suggestions
Some of the Arduino Pro Minis come with MHz and V markings on the back. In my case, none of these were checked. Make sure you check them as your new Arduinos arrive, so you won't have any problems identifying them later!


7216: <p>I have many Arduino Mini Pro, every from different seller.
Few are 3.3v and most 5v.</p>

<p>I had to clean table for Xmas and now I do not have any idea how to identify 3.3v Arduinos.</p>

<p>They do not have any marks. I bought them on ebay.</p>

<p>I know 3.3v has 8mhz clock but only one my arduino has big crystal with 16.000-30.</p>
 23751: <p>On my board the regulator is marked L0UA which is the code for a 5V regulator type LP2985, L0UB is also 5V. </p>

<p>A 3.3V regulator would be marked L0RA or L0RB.</p>
 73027: <p>Some clones on ebay labeled &quot;the simple&quot; come with 4B2X. They are 3.3v but the maximum voltage is 10v instead of 12v so beware to not burn it out.</p>
 3879: <p>The only difference would be the regulator and crystal (8MHz on 3.3V). As @IgnacioVazquez-Abrams suggest supply power to the "raw" and measure Vcc.</p>
 10350: <p>Check the label of the voltage regulator, KB50 for 5V and KB33 for 3.3</p>

<p><img src="https://i.stack.imgur.com/2CkXa.jpg" alt="Voltage Regulator"></p>
 368: <p>Plug it in and measure the voltage on pin 4 of the MCU.</p>
 15845: <p>The safe way (i tested it).
First connect to 3.3V USB-TTL output first (also 5V Mini can work with 3.3V).
Now prepare this sketch:</p>

<pre><code>void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(10000);
  digitalWrite(13, LOW);
  delay(10000);
}
</code></pre>

<p>Choose "Arduino Pro/Mini" with 5v/16MHz board and upload sketch.
If you see that the blinking is correct (10 sec ON/10sec OFF) - you have 5V/16MHz Arduino. Otherwise it will blink at 5 sec intervals instead of 10 sec.</p>
 6980: <p>The regulator should be marked K850(5.0V) or K833(3.3V).</p>

<p>A 5 volt part has a 16MHz resonator may be marked with "A1" or "A'N"<br>
A 3.3 volt part has a 8MHz resonator may be marked with "80'0"</p>

<p>As others have indicated, you can apply up to 12V at the RAW pin, and measure the output of the regulator.</p>
 25808: <p><strong>Consideration</strong> (go down for the actual solution)</p>

<p>If you have a lot of different China-Arduino-clones, the easiest way will be to just test it. The cloned Arduinos use a lot of different voltage regulators and often you won't even be able to find datasheets for them on Google.</p>

<p>Someone here provided an Arduino sketch which helps you identify the frequency with which the Atmel is running and the comments say "it is the safest method". But why? It is complicated (you need to connect the USB-TTL-converter to each of the different Arduinos and sometimes they have different pinouts, so it's error-prone - you could easily fry your Arduino if you connect the wires to the wrong pins)...</p>

<hr>

<p><strong>Solution</strong></p>

<p>I think the safest way is just connecting a voltage in the range of <code>6V-12V</code> to the <code>RAW</code>-pin and measuring the (regulated) output of the <code>VCC</code>-pin. </p>

<p>There is <em>no reason</em> why this would kill your Arduino (in case your Arduino isn't broken - which would be good to know anyway; and of course you shouldn't short anything).</p>

<p>I've done a short sketch of how you should connect your Arduino Pro Mini:
<a href="https://i.stack.imgur.com/vRb0Y.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/vRb0Y.jpg" alt="Connecting the Arduino Pro Mini"></a></p>

<ul>
<li><code>RAW</code> &lt;---> <code>6V-12V</code></li>
<li><code>VCC</code> &lt;---> multimeter (<code>5V = 5V Arduino</code>, <code>3.3V = 3.3V Arduino</code>)</li>
<li>One <code>GND</code> pins to the <code>GND</code> of the power source and one <code>GND</code> pins to the <code>GND</code> of the multimeter (you can take any of the Arduino pins labeled with <code>GND</code>)</li>
</ul>

<p>Don't worry if your Arduino looks a little bit different, just look for the pin labels. For instance, one of my 5V Arduino Pro Minis looks like this, but also has <code>RAW</code>-, <code>VCC</code>- and <code>GND</code>-labels (just one of the <code>GND</code> pins is at a different position, make sure you check your Arduino pin-labels):
<a href="https://i.stack.imgur.com/gge9q.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/gge9q.jpg" alt="Different Arduino Pro Mini layout"></a></p>

<hr>

<p><strong>Alternative</strong></p>

<p>You don't want to use the method above for some reasons? Then you need sharp eyes and some luck that the following list includes you components (some of these are collected from the other posts, some of them from my Arduinos):</p>

<ul>
<li>3.3V:

<ul>
<li>Voltage regulators: <code>KB33</code>, <code>S20K</code>, <code>F34V</code>, <code>L0RA</code>, <code>L0RB</code></li>
<li>Oscillators: <code>80e.</code>, <code>80'0</code></li>
</ul></li>
<li>5V:

<ul>
<li>Voltage regulators: <code>KB50</code>, <code>L05</code>, <code>L0UA</code>, <code>L0UB</code></li>
<li>Oscillators: <code>R160JAC6s</code>, <code>16.000-30</code>, <code>A1</code>, <code>A'N</code>, <code>A'a</code></li>
</ul></li>
</ul>

<p>(no limit or warranty for these details...)</p>

<hr>

<p><strong>More Suggestions</strong></p>

<p>Some of the Arduino Pro Minis come with <code>MHz</code> and <code>V</code> markings on the back. In my case, none of these were checked. Make sure you check them as your new Arduinos arrive, so you won't have any problems identifying them later!
<a href="https://i.stack.imgur.com/ZFngy.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/ZFngy.jpg" alt="Arduino Pro Mini markings"></a></p>
