It's that simple.

If you're late & you truly believe there is a chance you are pregnant, take a home pregnancy test or go see your doctor for a blood test. If you're having sex, you can get pregnant. 
If you're late & you truly believe there is a chance you can't be pregnant, go see your doctor. A late period with no chance of pregnancy could be a result of numerous things. 
I don't get all these "Do you think I'm pregnant?" posts. Most of us are not doctors. Use your common sense when it comes to YOUR health.