Hitting off speed has more to do with pitch recognition than skill.
To train this, learn the silhouette of the wrist at delivery. If the wrist is fat = fastball, skinny = off speed.
You can also add in tools to your training methods. Take a PVC pipe and hang it behind the thrower. This will allow the hitter to see and train the movement patterns of the pitch. Fastball stays straight with the pipe. Curve ball go up. Sliders/cutters go below. Remember if a curve ball is low let it go.
These things are covered in our timing and exit velocity class.