A roulette ball may have a difference in size even on the first sight it may not look like it.
It may be 19mm and 20mm or 21mm balls size small difference but they are different. When looking them in the pocket they both take about 1/2 of the pocket with. They both are light in weight and with bad jumps across the pockets which in average may be different. Going from wheel to wheel trying to determinate which ball is used is uncertain and it may take some time to compare them.
I position my eyesight as on the picture is shown with the red line, making the line from the edge of the top of the wheel to about 5mm to the end of the pocket. When the wheel is spinning and the ball is close to my position I can see the top of the larger ball, in the case of a smaller ball I can not see it.