The next time you put your helmet on, check the chin strap. When your mouth is closed, the chin strap should be tight enough that you can only fit two fingers between the strap and your chin. If the strap is too loose it may not stay on your head when you need it the most.
Chin straps can loosen over time, so it's a good thing to check a couple of times a year. Check out this video to see more information about how to properly fit a helmet.