How to tell if you banana is bad and know if you should eat it or toss it
Look at it:
Brown spots or black skin on bananas are actually quite normal and not a sign that they have gone bad. If you find mold growing on it or any kind of liquid seeping out from the banana, then the banana has gone bad.
Mold that grows on bananas looks similar to the mold you see on bread. If you see mold, its best to wrap it up and discard it. Make sure your kids or pets don't get to it - Some of the molds can cause organ damage. Because bananas are soft, molds spread through the banana so just cutting off the moldy part won't do.
Bananas that smell off, sour, moldy, or fermented are not ok to eat. Toss it or use it for your garden. Chances are that it has got spoilage bacteria growing in it - not harmful in small quantities but best not to risk it.
This is the best way to tell if it has gone bad. If you find that the inside is brown and very mushy- it might be time to try something else with the banana instead of eating it.
Bananas should be firm to the touch, but if it is really soft, squishy or has any kind of liquid oozing out of it - it might be time to do something else with the banana other than eat it. Soft and bruised bananas are prone to getting infected with molds.
Fruit flies will lay eggs inside the banana so if the skin has split and there are fruit flies on your banana, I would decide not to eat this one.