How do you know specific fruits and vegetables are unconsummable?

This seems to be the year of questionable produce. In light of that and to prevent unnecessary tossage I thought I would enlist the more experienced brains on this.

For instance I usually throw away avocados when I open them up and they are significantly shot threw with black lines. But if I were to open it up and leave it it would brown. If I covered it with lemon or citrus it would be perfectly fine. So is there something unhealthy about the color brown.

Banana’s I would toss in the freezer for various recipes so part of me wonders if this ok. But then I realize the acidity of a banana changes as it overripens, what are some of the other changes taking place?

If I open an apple and there were brown spots on it, I would trim the spots off. But if I cut an apple, didn’t eat it right away and had it in a bag in the fridge I would just eat the brown.

Mangos are another bag of questions. There are many other examples I didn’t include. Feel free to address these.