Cleaning, How To Detect

To a large extent, the best "sign of a cleaned coin" is that it doesn't look like a normal, uncleaned coin. So this to a large extent comes down to experience: handle lots of uncleaned coins, and a cleaned coin will stand out as being weird and different. But here are a few pointers for the most obvious cleaning types.

A heavily worn coin that looks mirror-polished and "shiny" has obviously been polished.

An old silver coin that looks mildly circulated but is completely tarnish-free has likely been dipped to remove tarnish.

Craters and pits in copper, bronze and cupronickel coins are where corrosion/verdigris has been chemically removed.

A generally rough pitted surface is a sign of acid cleaning.

A coin being "the wrong colour" is often a sign of old cleaning. Copper turns an unnatural pink colour if chemically cleaned with acid, and turns orange if cleaned with ammonia.

A coin with heavy tarnish in the deep crevices of the design, but is completely tarnish-free on the high points and fields, has likely been wiped with a polishing cloth.

If you see a nicely toned coin which looks like it has blobs of what appears to be black petroleum jelly stuck in the cracks and crevices of your coin, this is a sign that a coin has been cleaned and then artificially re-toned using Dellers Darkener - which is a mixture of petroleum jelly and sulfur.

Here's a thread on identifying whizzed coins. http://goccf.com/t/423682