Each of your hairs has its own very short life span. As long as you aren't going bald, a new hair will take the place of an older hair that has fallen out. Your hair grows approximately 1.25 centimeters or .5 inches every month as long as you aren't experiencing any major health issues. Pregnancy will also slow this process down unless you are taking vitamins. When you lose hair, which is natural, you can easily check to see where the new growth is.
Look in a mirror with the proper lighting overhead. Brush your hair out by parting it down the the middle and away from your face.
Check your roots right at the scalp. It's very helpful if you've had your hair dyed and haven't had the roots touched up, as you will be able to see exactly where the new growth is.
Check the very edge of your hairline above your forehead. Since you naturally lose hair every day, you would have lost some directly in front too. The new growth will be much shorter.