Do you use certified cleaning solutions and equipment?
Cleaning professionals have an array of solutions and equipment, some of which bear certifications from certain industry organizations. Ask the cleaner if they use certified solutions proven to effectively clean the carpet. Optionally, find out if they have a certification for using that solution.
Do you vacuum before deep cleaning the carpet?
That question is probably the most important one you should ask your cleaner. Generally, professional cleaners should vacuum the carpet before they give it a deep clean. How they respond to your question about vacuuming before deep cleaning should tell you about them.