Vegas is still the land of freebies, especially when it comes to alcoholic beverages. You can get as many free drinks as you want, but there is a catch- you need to be inside a casino and actively gambling.
All casinos cater to gamblers and furnish them with free drinks- both alcoholic and non-alcoholic. The moment you enter the gaming floor, you're being watched. Many of the newer slot machines have a program that records your activity. If you gamble for more than 30 minutes and continue to spend money, a cocktail waitress will come and ask if you want a drink. Once the drink is provided, tip the waitress at least $1, or you might not see her again.
The people who get the most free drinks are those who play table games. The cocktail waitress will usually provide a free drink to everyone around the table, and for those who continue to gamble, the drinks will keep on coming. Don't worry; the casino always wins because those who get drunk spend more.
The casinos are fully aware that to keep people in their seats and gamble, they have to do their part- supply you with a free drink. If you are just going to watch others gamble or play slot machines for two to three minutes, you're out of luck. In the past, non-gamblers would luck out with a free drink, but those days are coming to an end. If you do not play, there is no free drink.