There are a number of different ways to measure your seat height, from laser beams to anatomical estimates. Here are a few, summarized:
Heel method: at in the saddle, push your crank arm all the way down with your heel. At the bottom of the pedal stroke, your leg should be completely straightened, with your heel still touching the pedal.
109% Rule: measure your inseam and multiply this number by 1.09 (or 109% of your inseam). The answer is your seat height.
Holmes Method: film yourself riding on a turbo trainer with your feet in their normal position on the pedals. While analyzing the video, pause at the moment that your foot is at the bottom of the pedal stroke. Here, look for your knee angle, which should be 25-35 degrees.