Well, I don't know if you're going to like this answer...
The most common indication of squeezing the ball is a callous just below the crease of your thumb, or in the case of a large thumb hole...on the pad of the thumb.
When we first decide to try the sport of bowling, whether we are nine years old or an adult, we begin our experience with a house ball. The only way to hang on to a house ball is to squeeze it. The longer you bowl with a house ball, the more your muscle memory engrains the squeezing action into your delivery.
I've found that bowlers switching from a conventional grip (the grip used in house balls) normally require a transition into a finger tip grip. At first, the bowler needs to experience the change in delivery that allows the thumb to clear and the fingers to impart rotations. But they still squeeze the ball because they think they need to. Working with the Pro Shop
operator, the bowler needs to concentrate on releasing tension from the hand. I say that they need to work with the Pro Shop
operator because it is likely that the ball will begin to fall off early and require a pitch or span adjustment. It may just require tape.
I tell my clients that if they are going to squeeze the ball, do it at the top of the backswing, then release the grip as the ball travels forward in the swing. Centrifugal force will keep the ball in the palm of the hand until near the bottom of the swing at which time gravity will take over, pull the ball off of the thumb and the rotations are imparted by the fingers.
The exception...the grip-it-and-rip-it crankers. They have the 'squeeze-release' timing down so it occurs at the bottom of the swing in milliseconds.
Nothing wrong with that, it just takes a long time to master.
Hope this helps...