1. Stand over the bicycle's upper tube with both feet flat on the ground
2. Measure the distance between your groin/bottom crotch and the upper tube
3. If the distance is between 2-3 finger spaces or 1-1.5 inches then its a the right frame size for you.

Note:  Distance between the groin and upper tube differs according to different bikes. Road bikes need a clearance of 1-1.5 inches where Mountain bikes require a 3-4 inch clearance especially if the bike is going to be used off-road a lot. BMX's on the other hand come in one size for all ages as long as you have a comfortable reach towards the handlebars and peddles (same goes for female bikes).