Summarize:

Check this by making sure the end of the shoulder pads (the seams) end where your shoulders end under the jacket. If you are buying a three button suit the middle button should not fall below your belly button. If it's a two button suit the top button should not fall below your belly button. If you put your arms at your side, your knuckles should be roughly even with the bottom of your suit jacket. Check that the length of the sleeves is right. The sleeves of your suit jacket should sit at base of your thumb (where your thumb meets your wrist). The button up shirt under your suit jacket should be visible. You should have roughly 1/4th to about 12 inch (1.3cm) of shirt's cuff peeking out from the sleeves. Slip your hand into your lapel. If you hand is flat, it should slip in easily. If you put your fist into your lapel, the highest button on your jacket should pull. There should be roughly one inch of "break" at the bottom of your pants (where the end of your pants fall on your shoes). There are three options to the size of your "break." Full break is when the hem hits the top of the shoe's soles. This is a fashion forward look. Half break or medium break is when the hem hits roughly halfway down the back of the shoes. This is a traditional look. No break is when the hem ends right before top of your shoes. This is a casual look.
Make sure the suit jacket isn't too wide. Make sure that the proportions of the jacket are correct. Be sure that your suit jacket isn't too long. Make sure that the suit isn't too loose. Check that your pants are not too long.