I've always heard that manufactures have no way of conclusively discovering an overclock after the fact, damage or no. The warning of a voided warranty should you OC is a fear tactic. Sure there are signs of damage that can result from overclocking, but a faulty chip will make the same burn marks (rare) should it fizzle out, and a faulty chip is covered under warranty.
Futhermore, A GPU overclock is stored in an internal app, as the CPU's is stored in the BIOS. As soon as you pull that GPU/CPU out, it loses all indication of any tweaks, it becomes a static chip. Other than that physical damage (if any) that manufactures can guess at, nothing else that I know of can be used against you regarding the technicality of overclocking.