They cant. . . If you are smart about it.
However they have programs that watch these sort of things. Say I am talking to a buddy on yahoo and I have KK and he has AA. He raises, and I fold. Red flag will be put on your two accounts.
Now if you are using it on a smaller scale advantage. Lets say I have pkt 8's and you have K-8, there is no real reason to play my 8's anymore. A fold won't cause anyone flags.
Someone else may be able to explain a bit more in detail later.