Asking yourself: does your instructor show you techniques that other equestrian say are wrong ore unsafe?
Does he or she give conflicting messages, saying that it's okay to ride a horse this way now, in this situation, but does not or cannot explain why you should not ride that way in the future?
Does he or she insist that mileage will cure a lot of bad habits?
Do you feel nervous, unsafe or insecure on your horse or on the lesson horses?
Is this feeling taken seriously by your instructor, or are you told to "toughen up"?
If you do feel nervous, is it going away or increasing with time?
A good instructor will help you relax. A bad one will make you more nervous as time goes on.
Do you feel you look and ride differently than other people you see in videos or at shows?