I'm looking to learn traditional tai chi. I don't really know much about it, the only thing I know is I need it for self defense/subduing suspects (I'm a cop). I found one teacher in my area who is not teaching only relaxation tai chi. So how should I know if I'm gonna learn something useful from him?
Ask him. Ask him what style you are learning. Ask him who was his teacher. Ask him if he teaches the applications with it.
Watch him and his students. Are their joints stiff and locked? Do they have bad posture? Do they root or are they unbalanced. Do they look tense or relaxed?
Why did you pick Taijiquan as the art to learn to deal with suspects as a cop? Taijiquan is very effective in Qin na but I would think learning Police combatives or something like Eagle claw/Japanese Jujutsu would be more focused on teaching you joint locks.
Do not think you are going to learn Qin na right away, you want traditional Taijiquan well learning application should be the last thing on your mind if you really want to learn Taijiquan. To put it bluntly your teacher has to break you apart and put you together again. You need leg strength from standing post forms, You need to learn how to relax tension, You need to know how to do the form correctly, sinking and rooting. You need to go through Tuishou to understand the principles and theory needed to do the application. Don't expect to learn applications and Qinna till much later. With that said if you are in no hurry then the reward for your patience in Taijiquan will be great.
But for someone who needs to learn martial application and quick Taijiquan is not the way to go, you should look into Judo, Krav Maga, Jujutsu, Hapkido, Aikido, External kungfu styles.
As far as only teaching relaxation Taijiquan, well alot of people teach that because most people come to Taijiquan looking for that.
Maybe he knows application just noone asks him about it, maybe he only teaches it to students who have been with him a while.
Again why did you pick Taijiquan to learn application for apprehending suspects?