Screenwriting, as movies, are subjective. What one person likes, another person does not like. If you've ever discussed a movie with someone, you know what I mean.
The same goes with script consultants. What each one sees in a script might be different than what each other one sees in a script. So, you should find out what kind of genres/movies they like the best. If their interests fit the genre of your script, then, they will be more objective when helping you out with your script because they have a deeper knowledge of the genre and what works and what doesn't work with that particular genre.
Another thing to look for when choosing a good script consultant is where his/her focus is. If they have a background in working on high concept projects, then, their focus will be on high concept projects. They will be looking for "cookie cutter" elements in your screenplay. In other words, high concept movies are movies that are the same as other movies, the only difference being a unique and fresh approach to the same genre.
For example, with high concept action flicks, they will compare your script to successful box office action movies, if your script is an action script. The same applies to romantic comedies, animation, etc. The more elements they see missing in your script, the higher price they're going to charge you to help "fix" your script so that it fits the mold of a successful action script.
The problem with this is that the more they have you fit your story and characters into a successful box office mold, the more it takes your story and characters away from you, your original intention for your script. And, the more danger it puts your script in with respect to being tight, rhythmic, the appropriate tone and mode, and a smooth flow with respect to story, characterization, dialogue, etc.
So, choosing a good script consultant boils down to why you are writing a script and what to you want to achieve with it.
Box office success? Acclaim for its story and characterization? A combination of both? Whatever. The fine point of choosing a good script consultant is for you to know what your goal is with your script.
I hope this has been of help to you.
Donald L. Vasicek
Olympus Films+, LLC
The Zen of Writing