First of all, I think it important to be aware of what one is looking for in a Dom/me. 'Not treat you right' can also mean 'not compatible', and there are many ways and interests.
Example, a Dom who says "It is all about me" may be just what makes you heart tick, or it may make you feel real bad.
Also, as others have said, get rid of set ideas of how things 'should' be, but listen to your own inner voice. If the Dom/me in question does or demands something you cannot handle or which makes you really uncomfortable, I would say do not, but not, get into internal discussions about what a sub 'should' do, just listen to your voice and communicate the problem, or get out of there!
Maybe it need not be said to beware of the strutters, the "kneel bitch" types after 10 min conversation, and the types who claim that if you are a sub you just submit, period, or you are not 'for real'.
Another thought that comes to mind is that at times, when sub needs are really really pressing, it can be hard to think straight. Please hang on to your head, because the sharks out there are likely to recognize that condition and take advantage. Take you time to get to know people, however much your system yearns for a Dom/me.
The rest I think it sort of common sense and experience..I do not think you can always know.