>Find the card(s) that improve 2+ players hands at once, and increase the probability of that card being dealt on the river ever so slightly more than the remaining cards in the deck. How would they ever get caught?
In that case the river card would deviate from what should be dealt at random and with a large enough sample it will be caught.
One the subject of two players going all-in here is a reference of a study done by Steve Brecher for Stars' hands -
http://tinyurl.com/d5ji