How can I distinguish between a serious skin condition and something that can be treated with an antibiotic salve?

Skin related. Conditions are pretty tricky and difficult to self diagnose. I recommend you either see your internist at the minimum or a dermatologist. Good luck. Dr. B.
You can't. Believe it or not, it takes quite a bit of schooling and training to be good at it. Please see your doctor or dermatologist.