If your car is parked and you notice your tire is flat when you come back to your vehicle, it’s a good idea to inspect it for visible damage and–-if you can’t find any–-test whether or not it holds the correct pressure.
Should you find that the tire doesn’t hold the correct pressure, you’re probably dealing with a leak, in which case your tire is flat and not underinflated. If your tire is flat, you’ll know because the tire visibly looks underinflated and the car will be sitting unevenly.
A few key symptoms undeniably signal a flat tire while driving, such as a loud thumping noise followed by a flapping or slapping sound that gets louder the more you accelerate.