I could write a list three miles long in response to this post, but I'm going to take it from a slightly different angle, if you please...

I truly think that subs know how to spot the bad apples. It's easy to see an "arsehole" if you're looking. IF you're looking...

This is not to say that some good subs don't get blindsided now and again, but the significant majority of problems that I've been witness to occur because the sub isn't bothering to screen any further than his or her own very immediate desires. No one spends time to get to know each other, the sub goes into the relationship with the goal of having their own desires fufilled and then wonders why they were treated badly by a Dom looking to get only theirs fufilled as well.

Communicate.

Can't stress that enough - spend some time determining what a D/s relationship means to each one of you. Truly, if you don't have the time to do a relationship right the first time, do you have time you pick up the pieces from doing it the wrong way?