I just finished 10 days of amoxicillin & had diarrhea d/t it how long till it's gone or is due to loss of good bacteria from body & how do I tell?

Possible. It is very possible your intestinal tract flora have been disrupted by the antibiotics. An easy way to help restore your "good" bacteria in the gut is to eat yogurt with live cultures. The diarrhea should resolve soon; please be sure to stay well hydrated with clear fluids. You can use products such as pedialyte to help keep your electrolytes up.
Post-antibx diarrhea. Many antibiotics are non-specific as to what bacteria they kill. When your normal gut flora is depleted by antibiotics you may be at risk for opportunistic pathogens. A very common one is clostridium difficile (c.Diff). Your doctor, in considering this possibility, will order a specific stool check. If c.Diff is found treatments include: different antibiotics, toxin binders, florastor probiotics.