The first sign that you might have a broken thumb is the severe pain that accompanies a bone fracture. According to Reference.com, you may feel numbness in addition to the pain. Swelling and bruising in the area is common.
The pain that comes with a broken thumb isn't always the same for everyone. As Reference.com describes it, the pain may be localized at the site of the break, or felt as a deep throbbing that radiates all the way up your arm. It's the same for the swelling, which may affect one area or your entire thumb, and could start right away or take several hours to manifest. Bruising that accompanies the break is sometimes a sign of internal bleeding at the site.
WebMD adds that people who experience a fracture to a finger often notice that the digit is bent at an odd angle or otherwise deformed. Trying to flex or bend a broken thumb is usually extremely painful, but don't be fooled by a finger that has some range of motion and dull pain. The only way to know for sure if your thumb is broken is by going to the emergency room for an X-ray and appropriate treatment.