I struggle with this myself. I found an article about cleaning coins on Omnicoin that helped me out somewhat. It's not so much about how to spot them, but about different methods used to clean/alter coins. That gave me more of an idea of what to look for. Forum Mom told me I could link to the article, but I am too computer illiterate to do so. Look on the Omnicoin website under Numismatic Articles for The Truth about Cleaned Coins. There have also been discussions here about cleaning, you might try an archive search.
In any case, it is very easy to be fooled by an auction picture. I won't bid on any coins that don't have a large enough picture to clearly see everything. I avoid coins that have hairline marks, or ones that have dirt encrusted around the fine detail(stars, numbers, etc) with the rest of the coin being clean & shiny. I also am wary of coins that don't seem to be the correct color for the amount of wear(worn cents that are bright red, bright silver coins with no detail, etc).
You can avoid some problems by buying slabbed coins, but only if the grading company is reputable. That means PCGS, NGC, ANACS & sometimes ICG. It's a crying shame that people will sell you coins that are overgraded, or cleaned, or whatever, just to make a profit. Some coin dealers are just like some stock brokers, or used car dealers, or contractors, or.....well, I'm sure you get the picture.