How to tell if a carbon frame is damaged or if the paint just has stress cracks
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.