I've seen people asked to leave SAB, the Polynesian pool, and the BW pool. As another poster stated, either security or a lifeguard will ask for ID's, and then ask the party to leave if they don't have them. They will linger near them to make sure they do leave.
One cold January morning, DH and I wandered down to the theme pool at the GF and were asked for ID--we were in street clothes with no bag that we might have bathing suits in. We were staying at the hotel, so no biggie, but they were obviously enforcing the rules, and there wasn't even anyone in the pool!