If you're playing a slot machine, playing at a card table, betting on a roulette table, or any other gambling in the casino, free drinks are available to you. However, some casinos will offer free drinks to all guests, whether they are gambling or not, as long as they are 21 years of age or older.
The more exclusive casinos on the Vegas Strip like the Wynn, the Venetian, and the Bellagio usually offer more free drinks for players than other hotels. If you hang around in the less exclusive casinos like the Flamingo or O'Sheas, don't expect too much.