Unfortunately not all coins sold are properly annotated so buyers should be on their toes. One type of cleaning involves what is referred to as "dipping". Dipping involves a chemical cleaner with the result of a cleaner looking coin but with less luster. Coins improperly dipped typically leave a white film on the surface of the coin. One of the best tools I've discovered for this is to use a lighted loupe. The bluish/white LED light on loupes or magnifiers really brings out the white film to make detection easier and it makes it easy to point this out to a seller who will probably lower his/her asking price.
The next level of cleaning is much more severe. As coins tone or tarnish this is not dirt but the actual silver or copper material so when you remove the toning or tarnishing the actual silver or copper is being removed from the coin. If the tarnish or tone is light then damage is minimal but if its heavy then the composition of the coin and design change drastically. Look at this Australian large copper coin I grabbed for a dollar just for this article. It was bought at an online auction and the picture was zoomed so far out that you couldn't inspect the coin. A clue that this was a cleaned coin was by its color. Most large copper coins tone very quickly to brown or even a chocolate dark brown color, being such a bright copper color signaled it was a recent cleaning and since the coin was from 1919 my hunch was toning was pretty severe before the cleaning.
After receiving the coin there were obvious brush marks on the surface of the coin confirming cleaning suspicions. After zooming in on one of the cleaned areas of the obverse the real damage can be seen. The surface is dotted with pits where material was removed, so this coin was first chemically treated and then brushed. The reverse was no better, it showed the same brush marks and pitting. To determine what this cleaning did to the coin it was thrown on a scale. Most large coppers from this period weigh 9.3 to 9.5 grams but after a thorough cleaning it's weight dropped down to 9.1 grams removing anywhere from .2 to .4 grams of copper. So the question at hand is "is this good enough for a collector"?
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The answer is maybe. The material removed is not obvious without magnification but the tremendous amount of brush marks in the coins fields are quite a distraction. For myself I was simply looking for a single example of a George V design and I believe a little aging and toning may diminish the the ugliness of the brushed fields so it barely squeaks by as an acceptable piece but I will annotate it as "Heavily Cleaned".