If it is truly an abscess, it'll need to be drained, they do not go away with antibiotics alone. in fact some studies have shown that simply lancing the abscess and allowing it to drain is sufficient. What everyone has said about symptoms is correct. Abscesses are hot, red, swollen, and extremely painful. Mind you, this also describes any inflammatory response.
If you're concerned it's an abscess, GO TO THE HOSPITAL. ER docs don't care how it got there, the majority of abscesses are idiopathic (no known cause). What they'll do is stick a needle in it to try to express pus, and if there is, they'll lance it with a scalpel (after injecting local anesthetic, of course). It may or may not get packed, and you may or may not get antibiotics (usually a first-generation cephalosporin).
Anyway, whatever you decide to do, be sure to use sterile technique with all future injections.
Take care.