From VeloNews' Lennard Zinn:
"Inspect all tubes for cracks, gouges, buckles, dents, and paint stretching or cracking, especially near the joints where stress is highest. With a carbon frame, use the "coin test" to check for damage to underlying carbon layers. Tap on the tube with a quarter in the questionable areas and compare it with the sound on other tubes, in surrounding areas, and on the opposite side. If you have delamination or cracking in underlying carbon layers, especially in central areas away from the joints, you'll be able to hear the difference; the damaged fibers deaden the nice "clack" sound you hear when tapping on an undamaged tube. If in doubt, take it to an expert for advice. Carbon structures that look good on the outside shouldn't get softer unless the layers start delaminating, and the coin test may be able to detect such areas."