user1622: I'm looking for a way to check the air-flow from an aquarium air pump. I need something that can test whether air is flowing or not and I'm not bothered about measuring flow speed. The air will be flowing through a pipe but if required I can have the sensor on the end of an additional pipe just used for measuring.

user8142: Flow sensors tend to be expensive. However, there are many pressure sensors that provide a simple analog voltage input and others with easy to use digital interfaces. 
As Igor Stoppa points out, these can be used to infer flow. Some can be purchased with tube ports that allow the user to mount the hose directly to the sensor. This would only require a "T" connector and some additional hose to connect to the air pump.
Aquarium air pumps may be pulsatile. So be sure to use an average pressure. 
Average Pressure:

above a certain point may indicate blockage
between certain values indicates correct flow
below a certain value indicates no flow

.

user4143: Grab yourself something like this

Butcher it to get inside it, and add three small magnets to the fins (should be simple enough - just keep it balanced).  Then glue it all back together again.
Add a hall effect sensor to the outside, and Bob's your uncle.
Or, since the principle of that unit is so simple, you could make your own from scratch.

user7856: Did you consider a barometric sensor deployed inside the pipe?
The pressure should be higher when the pump is active.

1622: <p>I'm looking for a way to check the air-flow from an aquarium air pump. I need something that can test whether air is flowing or not and I'm not bothered about measuring flow speed. The air will be flowing through a pipe but if required I can have the sensor on the end of an additional pipe just used for measuring.</p>
 8142: <p>Flow sensors tend to be expensive. However, there are many pressure sensors that provide a simple analog voltage input and others with easy to use digital interfaces. 
As Igor Stoppa points out, these can be used to infer flow. Some can be purchased with tube ports that allow the user to mount the hose directly to the sensor. This would only require a "T" connector and some additional hose to connect to the air pump.</p>

<p>Aquarium air pumps may be pulsatile. So be sure to use an average pressure. </p>

<p>Average Pressure:</p>

<ul>
<li>above a certain point may indicate blockage</li>
<li>between certain values indicates correct flow</li>
<li>below a certain value indicates no flow</li>
</ul>

<p>.</p>
 4143: <p>Grab yourself something <a href="http://rads.stackoverflow.com/amzn/click/B002VBW2I4" rel="nofollow noreferrer">like this</a></p>

<p><img src="https://i.stack.imgur.com/sSmLc.jpg" alt="enter image description here"></p>

<p>Butcher it to get inside it, and add three small magnets to the fins (should be simple enough - just keep it balanced).  Then glue it all back together again.</p>

<p>Add a hall effect sensor to the outside, and Bob's your uncle.</p>

<p>Or, since the principle of that unit is so simple, you could make your own from scratch.</p>
 7856: <p>Did you consider a barometric sensor deployed inside the pipe?
The pressure should be higher when the pump is active.</p>
