Is is possible to know if a CPU has been over-clocked in the past?

I would like to use a second hand CPU but I would like to know if it has been over-clocked by the previous user.

Is there a way to find out about this piece of information?

Does clocking back down leaves a detectable trace or signature somewhere?

PS: I know how to find out whether a computer is currently over-clocked.

I think your best bet would be to ask the previous owner. Overclocking is done from the motherboard and if it has been detached from a motherboard there shouldn't be anything stored on it, they're fairly 'dumb' in that respect, it's a processor not storage.

If you have the motherboard that it was in, and no other processor replaced the one in question, you may be able to look at those settings in the BIOS.

No. Your best bet is to ask the seller. CPU's can't tell you if they have ever been OC'd. Checking the bios of its running system could tell you if it currently is, but there's no way to verify for sure that it has or has not been OC'd. That said, your odds are good that you're getting a good CPU as long as it is working now.