I was working the "Eye" in a casino in Wisconsin and we caught on to two guys colluding... after another player requested one show his mucked hand. These guys had been raising back and forth all the way down, and the loser had mucked some random BS, with no draw or pair. Seemed suspicious, so they kept asking to see their mucks, and we banned 'em both about an hour later.

It does work, though it's the only poker room collusion we caught in my 2 years there.