Definetely check the valve stems like everyone else says. I doubt your car has tire pressure monitors. One other thing you could check is when you first start the car up, and all the lights on the cluster turn on to show they're working, look for a symbol of the bottom half of a tire with an exclamation point in the middle of it. That's usually what it looks like, I'm not one hundred percent sure. Some late model GM cars use what's called a passive tire pressure monitoring system. It uses the wheel speed sensors to detect small differences in wheel speed due to underinflation of the tires. Hope that helps.
