Bacteria vs Virus?
It's all a bit of a guess.
Empirical tests, (swabs, cultures etc) are generally not useful. (The labs tell us this.)
The clinical findings (listening with the stethoscope, looking at the tonsils etc) add something to the picture, but again, don't always give the answer-very rarely does a "barn door" case turn up.
Predictive tools (eg The Centor Criteria for sore throats) give us some guidance.
Factors also involved in whether or not a patient leaves the surgery with or without antibiotics include how late you're running, patient expectations and other medical conditions the patient may have.
My eldest has had two courses of antibiotics during her lifetime, the youngest (now a teenager) has had one.....not because I'm a bad dad, just because I believe that antibiotics would have made little difference to their recovery.
Of course, the caveat to this post, is, if you are worried about breathing, alertness, hydration, rashes or the duration of an illness, then seek medical advice.