How do we know if the coin is cleaned? 0

[Glowecco]

How do we know if the coin is cleaned?

[JKK]

One looks for large numbers of tiny scratches, either rotary or horizontal, from abrasive cleaning. One also looks for chemical residue that didn't get cleaned off. Most of the time, cleaned coins just look wrong. A good example would be a blast white Mercury dime that is worn mostly flat; you don't even need to look closely to tell that it was cleaned, because the cleaning is what caused that bright white color.

[ilLOminatus]

It's a hard question to answer, the best way to learn is through experience. But otherwise JKK answer covers most of it.

[jgrinz]

On OFF look to toned areas also 

[Matt_dac]

Symmetrical small scratches are a dead giveaway but it can be hard to detect sometimes.  I am very selective in what I buy raw and rely on the pros for anything of high value (professionally graded and encapsulated).