People that have been fucked over, and maybe even more so, for people who've provided a shoulder to others that have been damaged, tend to build a list of flags/signs/patterns that indicate bad behavior. We all do it to some extent, it's only natural. It's also natural to add to, or modify, that list by applying personal pet peeves and hot buttons.
Some people become a walking, talking, early warning system, but as someone once said, if your looking for trouble, you'll find it.
Keeping in mind that every person and every situation is unique, will temper the temptation to condemn everyone that sets off your alarms. After all, those alarms should be cautions not automatic convictions.
In affairs of the heart, use your head, but don't discard your gut. The gut feelings, sixth sense, vibes, what ever you call it, are a valuable resource when balanced with rational/logical thinking.
Besides, it's better to have loved and lost, than never loved at all....as long as you practice damage control.