What would a broken thumb look like?

Depends: Thst depends on how bad the break is. If a bone is just chipped in a small area or even have an avulsed or torn tendon, the bone may look normal but jist not move right, while a large break may be comminuted and very misshapen. If there is a question, if you can't grip or squeeze things then you should get an x-ray.

It swells: Thumbs injuries are bad. We use the thumb to grip and it has been said that the thumb separates us from lower life forms. If you have injured yours and think it might be broken see an orthopedic surgeon right away for an eval.