Your forearm and your arm hurt really bad but how do you know if it's sprained?

Well, a break could be one of two things, a fracture, or a break. A break would mean it is out of the skin, and a fracture is just a crack in the bone. A sprain is just a muscle over extension, there is usually just swelling and it will feel better in a few hours - days. The only way to 100% sure, is to get and X-ray. If you don't have the money or time, just ice it, and if it STILL feeling like its on fire, after a day, then there is a good chance it is broken.

Hope this helps!