The effects of steroids and their benefits for training don't end when you cycle them off.
The ability to train more/recover from injuries quicker etc etc are massive bonuses to using steroids, and also Any strength/speed gains will still be there come fight night.
If levels are above normal, it's not TRT. The "R" means "replacement". Anything higher than that is "roiding" or blatant cheating. However, if levels are normal during the whole time, the abilities you mentioned should be equal to a person not needing TRT, right? So random tests would be enough to grant they are not over using it, therefore going out of the scope of a Replacement Therapy. Hell, diabetics are more than often having daily blood tests. I am sure they can easily control fighters.