Its very simple and easy. Look out for these three things out of all others:
Without any of these three in a relationship, that relationship is doomed for failure.
If your partner repeatedly behaving in a manner that would make you suspicious of cheatings. That relationship is doomed to fail.
If your partner talks down on you, and intend to embarrass you infront of people. That is not a healthy relationship.
If your partner doesn't show any support in case you're in an emergency or in some sort of trouble, they don't care. That means you should leave that relationship quickly.