Markus Storck, lead engineer and owner of German carbon frame powerhouse Storck Bicycles, told me at a conference about 3 months ago that the best ways to tell if a frame is cracked are movement and time.
Movement, because a crack will flex if it's through the paint into the carbon, and you put pressure on the center of the crack, and time, because a crack will grow with time if it's more than paint.
If you are even halfway concerned with a crack, don't buy the frame.
It's just not worth the risk, unless you can afford to be wrong about the frame's ride-ability.
I hope that helps.
bicycles.stackexchange.com/users/1259/zenbike
Why don't you take the bike to a Cervelo dealer and have them check it out?If
the seller of the bike is being honest with you
it shouldn't be a problem
>
Last year I had a horriffic crash with my Trek Madone going over 20mph and the only damage my frame sustained was a small chip in the frame.
otherwise the frame is perfectly sound.
bicycles.stackexchange.com/users/3632/Charles Graham
If you can't feel it with a fingernail, the crack probably isn't in the carbon.
Another trick is to whack the tube with a quarter over and around the crack.
Do the same at a similar location that doesn't have a crack.
If there are any differences in sound, the frame has probably sustained damage.
For what it's worth, I bought a bike with very similar circumstances in December.
I used a piece of tape to mark the beginning and end of the crack, so I can watch for any expansion.
No movement so far.
bicycles.stackexchange.com/users/1442/Stephen Touset
The BB and head tube are some of the thickest carbon parts.
Listen to sound differences around it, but I think these areas would be some of the last places to crack.
Most cracks would occur either at a junction, thin spot, like middle of the down tube.
Also, if the crack you see has no marring, chipping or other damage on either side, its probably superficial.
Cracks caused by outside forces (impact) would leave other evidence on the paint, like scuffs, chips, etc.
bicycles.stackexchange.com/users/3124/Matt Adams
Have a look at http://www.carbon-bike-check.com .
AFAIK there is no other way to find structural issues in carbon frames.
Everything else is just guesswork.
Even a competent dealer cannot look into the fibers.
So if the frame is checked on the surface only, i wouldn't buy it.
bicycles.stackexchange.com/users/3655/Joerg Pacher
I am looking at a carbon framed road bikethat has me concerned.On the topside of the bottom bracket,drive side there is a very small hairline crack in the paint.The seller states that it has been there for two years with no change.He was told by the dealer that it is just in the paint and a fairly common occurence.Anyone else experienced this?It is a three year old cervelo.The crack is 3/4 inch long and you can't feel it with a fingernail.
How to tell if a carbon frame is damaged or if the paint just has stress cracks
bicycles.stackexchange.com
<frames><carbon>
bicycles.stackexchange.com/questions/8472
bicycles.stackexchange.com/users/3083/mikes
It is difficult to tell if a crack in a carbon frame is in the paint only or has gone into the carbon. If you can't feel it with a fingernail, the crack  probably  isn't in the carbon and if the crack you see has no marring, chipping or other damage on either side, its probably superficial. Cracks caused by outside forces (impact) would leave other evidence on the paint, like scuffs, chips, etc.  However, If you are even halfway concerned about a crack, you should not buy the frame. A crack in a carbon frame may be serious without being obvious. You should have it checked properly by a certified dealer or not risk it at all. A deeper crack will flex and grow over time, so use pressure to test for movement and tape for growth over time. Cracks in the carbon can usually be felt with a fingernail, or will sound different from other areas when tapped with a coin. Structural cracks usually leave evidence on the surface, especially when caused by outside forces.
A crack in a carbon frame may be serious without being obvious. You should have it checked properly by a certified dealer or not risk it at all. A deeper crack will flex and grow over time, so use pressure to test for movement and tape for growth over time. Cracks in the carbon can usually be felt with a fingernail, or will sound different from other areas when tapped with a coin. Structural cracks usually leave evidence on the surface, especially when caused by outside forces.