How Do I Know If My Coins Have Been Cleaned?
If the coin has been cleaned with an abrasive, the coin will have hairlines. Have a look at the coins in Overton's half dollar book; a large proportion (maybe 20%) of them appear to have been harshly cleaned with an abrasive. Also, abrasive cleaning often leaves some crud in the recesses of the coin (untouched dirt or left over abrasive).
If the coin has been dipped, it may or may not be detectable. A bright white 1801 half dollar is immediately suspect. Although it is possible for such an original coin to exist, it is unlikely. Also dipping can strip the lustre off of the coin, with the end result that there is no lustre where you would expect it to be for a coin in said condition (XF and better coins).
A natural coin has a particular appearance which reflects the history of its storage. Haphazardly stored coins tend to have a "dirty" appearance to the toning. Coins that have lived for a long time in a coin cabinet tend to have spectacular colored toning. Coins stored in a clean metal vault (such as an old style "piggy" bank) may stay white (or red) for a long time. Coins stored in albums develop either the familiar "ring toning" (slide type albums) or the much less desireable "one sided toning" (all cardboard albums). Coins stored in mint bags often show spectacular rainbow toning, similar to that seen on coins stored in coin cabinets.
Copper/bronze/brass coins that have been cleaned have an unnatural color, often looking like a toned gold coin. Even after they retone, they tend it tends to be uneven and a slightly odd color (watch out for dark areas). See that red in the recesses of that VF copper coin? Not a good sign! Naturally toned, *circulated* copper tends to be very uniform in color, although they might be dark and dirty around the lettering and similar protected areas. Uncirculated copper may tone very unevenly (especially proofs), so do not automatically count this against such a coin.
Exactly the other way around, silver coins that have been cleaned tend to be extremely uniform in color after they retone, including the tops of the letters and protected areas. Silver coins with natural toning will usually show some variation in the color at these places. Be aware that a uniform slate gray color can be produced on silver very easily with a number of chemicals. Finally, a heavily toned and subsequently dipped silver coin will tend to have a gray appearance caused by surface roughness rather than tarnish. This can be detected by careful examination with a strong magnifier.
The ANA advises that sudden "hard line" changes in color do not occur on naturally toned coins. Naturally toned coins exhibit a
gradual change in color or darkness. In any event, its mostly a matter of looking at a lot of coins and forming your own opinions.
Assuming that you are buying coins for your personal collection, in the final say, it is your opinion
that really matters.