2 seam fastball aka sinker

I have been working on my sinker ball…i practice on my own against the wall. At times I notice the ball sinking downward before it hits the wall. Now how would I determine if this is for real nad not just my imagination?

Any tips on the griping or anything else.

Thanks so much

Find someone to throw with or even just stand behind a fence when you throw or set up a camera behind a fence (on a bench or chair) and video tape your pitches and watch it after.