There are hints that people give off of their true natures.

1)Men who abuse/belittle service people (waiters, waitresses, store clerks, etc) by being pushy, rude or critical of them... are to be avoided if you want a considerate partner. These guys will often call others idiots, or stupid or worse to make themselves feel superior.

2)Men who tease excessively children, pets (dogs and cats), and laugh while doing so, are to be avoided too. Men who abuse animals are sadistic and should be avoided at all costs.

Often potential partners can summon up some nice acting for YOUR benefit, but they often don't keep it up in the rest of their lives, so if you witness disrespect to others or animals, LISTEN to that as a warning for yourself.

About 70% of narcissists are men, so this is a common trait. These guys are only interested in themselves and what YOU can provide for THEM. They will turn on you with anger and sometimes cruelty or violence if you disappoint them.

It takes a long time as others have pointed out here to discover the hidden traits of some men. I had the misfortune to have a father to teach me first hand how interpersonal sadism occurs in a home situation. So I have zero tolerance for this in my adult life.