You can't. In my experience, it's a crap-shoot. You can get a world of glowing reviews and ultimately find that even the "professionals" have their inherent biases and they are usually titled towards the mother, as they often are in every other aspect of high-conflict divorce and custody.
It's all about the interview process and one of the top few questions should be...
- How often have you made a recommendation that favor
shared parenting (equal or close to it)?
- How often have you recommended in favor of mothers?
- How often have you recommended in favor of fathers?
The answers to those three questions will usually tell you where you stand, assuming they'll be honest with you about it.