Before every ride, make sure your tires are in good condition. To check the pressure, you usually just squeeze them by hand. You won't get a perfect reading with it, but you'll soon be able to determine if you need to pump up your tires. Getting really nerdy might lead you to invest in a pressure gauge, which can very accurately measure the pressure in your tires. You can do the same on a road bike so you can find what the best pressure for your riding is. It’s particularly helpful for mountain bikes, since a few psi can make a significant difference in handling and grip.