To know if your tires have the right amount of air in them, the easiest way to check is to look for the warning light on vehicles that check tire pressure automatically for you. If you don't have a vehicle with this feature, either go to a gas station that has an air pump that checks the tire pressure for you, or use a tire gauge.
Checking your tire depth is just as easy. Simply take out a penny and insert it into the tread. With Lincoln's head down, if you can see his face, then you should have your tires changed.