There are a few ways to beat the slots in Paper Mario. The first way is to use items that can help you win more money. The most common item that can help you win more money is the Lucky Gold Star. This item will give you a chance to win lots of money every time you play the slots.
Another way to beat the slots is to use special abilities that Mario has. For example, Mario can use his fire breath to set things on fire, which will help him win more money. Finally, using coins that you find along the way can also help you win more money at the slots.