How can you tell if a therapy is questionable?

Sounds Too Good. Like anything else in life, if it sounds too good to be true, it probably isn't. Research is your best weapon. Ask questions. If your doctor is unwilling to, unable to, or avoids answering, get a second opinion. The internet can be both good and bad, so try to stick with approved medical websites.
Good rapport. The single greatest indicator of successful therapy is good therapeutic alliance or rapport. If you don't feel you and your therapist are working together on mutual goals for your improvement, you should look for another.