Most late model vehicles have a tire pressure warning indicator lamp. If this lamp is coming on after the vehicle has been sitting overnight or on cold mornings and it goes out after a few miles of driving, then your tires are getting low.
Usually, the low tire pressure lamp will come on if there is a 15% variance in pressure between the tires. Each tire has a tire pressure monitor in it. These sensors operate by battery. Battery life is usually 5 years. A low battery can also cause this lamp to come on.