If you have a specific problem with your dog, ask trainers what their experience is with this problem. Ask if they have experience with your breed. Ask questions if you don't understand their program or if something doesn't sound right. Where possible observe the trainer with other dogs before enrolling. If an instructor won't allow you to observe them, look elsewhere.
Instructors only spend a short time each week with the students' dogs.
Interview the instructors you are considering. Ask where they got their experience. Ask about failures as well as successes. Be skeptical of instructors who offer guarantees. Not all dogs and owners can be trained to the same performance standard in the same length of time.