Educate yourself about screenwriting and the movie business. You should know about story structure, pitfalls, market trends and genre conventions. The more you know, the less you pay the script doctor to teach you.
Ask about a script doctor's education. A Masters of Fine Arts or English degree shows knowledge of story structure. A consultant with a degree in the social sciences can help you know your characters better.
Check out the scripts the consultant claims to have doctored. You're looking for a match in style. You may like the action thrillers he doctored, but if your script is a comedy, you should look elsewhere for help.
Find out about her reputation. Search film industry news sources for articles about her and the films she's worked on. Assess if he seems more anxious to make a name for himself than to help you make your script the best it can be.
Have respect for the script doctor you choose. She may have extensive experience with the type of script you've written, but if you saw some of them and hated them, it'll be hard for you to take her advice.
Trust your instincts. You must connect with your script doctor with this large and complex project. A characteristic that irks you like arrogance, impatience or condescension will make you feel you wasted your money.
Consider hiring a consultant approved by the Screenwriters Federation of America (SFA). They'll likely charge a high price for their services, but as long as you gel with their personality, it'll be money well spent.