Recognizing the pitch type can be tricky and isn’t something you will get 100% correct all the time.  Key indicators can include spin of the ball, a pitcher’s body position at release, their hand/fingers/wrist movement on/after release, location of the catcher and path of the pitch.

This is something that you will get better at the more you focus on these aspects and can be a useful training exercise for hitters: to sit, watch, and recognize pitch type.