Old carbon the types they used 10 years ago is dubious with serious racing it's fast worn out.
Newer bikes have better technology, the new Carbon might not be jet on the durability level of steel(some types actually are) but it lasts for a reasonable amount of time.
Ride the bike check for strange noises and check bracket stifness(this applies to all frames not only Carbon)
Important is it a fairly recent bike? an by who and where was the bike used(also applicable for al bikes you might buy)
On thing you shouldn't be afraid for is bending as Carbon rarely bends the force needed to bend is only a little les then the one to break it(and this you will notice of course)For steel and aluminium these forced are further spread so there might be some bending especially from down tube due to riding in potholes, or against solid objects(dents are mosttimes approx 5-10 cm from steerer tube on the down tube, a frame with such a dent is 95% off the time wortless(this is something too look at with steel and especially alu)