There are several places that bicycle riders can find tire pressure charts for bike tires, including the maker of the tires and sites devoted to the sport. The charts provide a maximum and minimum pressure for different tires. While these are accurate estimates, the load of a bike and the conditions it is being ridden in also need to be taken into account.
Tires that are consistently low in pressure tend to wear out quickly, but also act as better shock absorbers than highly inflated tires. Wider tires that are found on mountain bikes generally have lower pressure recommendations from the manufacturer.