Take a regulation ball and place a wide black ring around it with a sharpy or paint stick. Hold the ball in a four-seam grip with the black stripe running parallel to the two fingers on top of the ball. (You need to locate the stripe / ring so this is possible.)
Go do some long toss and experiment with what you have to do to get the ball to spin such that you can see the stripe in the 12-6 position and reasonably clear. Wobble will blur the stripe.
You can find weighted balls with the mentioned stripe at the Frozen Ropes site.