How does one tell?

I've had several coins, mostly Morgans and Peace dollars, 1 $2.5 gold coin and 2 Washington quarters (1932 D and S) returned to me ungraded, from NGC with the disqualifer of "improperly cleaned". I did not clean these coins, so how does someone know when a coin has been cleaned? This has been a very frustrating experience, and is discouraging me from buying any older coins. My number of improperly cleaned coins is approaching 10-12, and is starting to cost me in wasted time and money (not to mention disappointment). I am now fully aware that the number of unethical coin dealers must be large. Any advice? Thanks.