How to tell if my thumb is infected?

See some symptoms. Is it swollen, is it red, is it painful? Do you see any red steaks emanating from yor thumb? If you've answered to the affirmative to any of the question, sees doctor.
See below. This is a problem that necessitates a face-to-face meeting with your doctor. This will allow him/her to examine you, ask specific questions. And possibly order tests to find out what's wrong and what to do to help you.