How is good bacteria overgrowth in the intestines distinguished from bad bacteria overgrowth? should i see a gastro for my suspicions?

You get sick: good bacteria are always in your colon making digestion better, bad bacteria when too many make you very ill

Microbiota, or: intestinal bacteria, need to eat. Evidence indicates that beneficial bacteria, which help you feel good and function well, prefer nutrients called prebiotics, which are found in high-fiber foods including onions, garlic, berries, artichokes, and many greens. Bad bacteria, which can make you feel bad and not function well, prefer sugars and fats found in processed foods. See comments for more:

Multiple ways: There are multiple ways this is determined. While not commonly done, a quantitative aspiration of bacteria from the small intestine can be done. But hydrogen or methane breath test and D-xylose testing can be also done. Also very common is seeing a patient's response to an empiric(trial) course of antibiotics; if one improves with antibiotics that it suggests overgrowth.