You can determine the size of a bike frame by measuring a certain part of the frame itself, depending on what type of bike it is. The website Ebicycles provides a bike measurement tool that varies depending on what style of bike is used.
For mountain bikes, measure the distance from the center of the crank to the top of the seat tube. Measure European road bikes using the distance from the center of the crank to the center of the intersection of the seat tube and the rest of the frame. BMX bikes are measured by the top tube only. Children's bike sizes are measured in wheel diameter instead of frame size.
Measuring the bike frame can help determine if the bike is a proper fit. Bike sizing guides are available, and they vary between road bikes, mountain bikes and children's bicycles. In order to figure out what size frame is right for you, record your height and inseam length; inseam is the more important of the two. You can also visit a local bike shop for help with measurements and testing. If you have your desired bike on hand, stand over the frame, adjust the seat height and level, and adjust the handlebars to find out if you can create a comfortable configuration.