How do you know when people go just plain rotten?

Go by the words they use, and the actions they take.  If they respect you, they will not use  abusive language or act in an obnoxious or obscene manner toward you and those you love. 

 If they don't care anything about you, they will verbally abuse you and mentally & emotionally disturb you and eventually physically make your life unbearable.

Njoy's answer is pretty much as good as anything I could add. But people just don't suddenly turn bad. There are indications, as Njoy mentioned, that should be warning flags. The truth is, some people just seem to be miserable SOB's their whole lives.

The best way to tell if someone is just plain rotten is to use your nose.  Do you smell marijuana?  Last night's booze?  BO that they haven't washed off in the last nine days?

The way to tell a good man from the rotten bunch is to use your eyes.  Does he have on clean company coveralls and a hardhat when he comes in for lunch or dinner?  Do you see greasy gloves that show he has been working like a real man?  Do you see tattered legs on his jeans that show he is a real logger instead of one of these slack body lowers just pretending to be?

The rottenest cusses I ever had to deal with were leftist lowers that try to tell us how to be way up here.