Put your measle ball between two other balls on your table. Take a straight edge, or use the rack and place it on top of the balls. The largest ball should show up easily by rocking the straight edge up and down by the ends like a teeter-totter.
I play with 3 or 4 guys who always insist we use their measle balls when we play.
On every occasion, I have done exactly what you described above to show them that their measle cue ball is larger than the rest of the balls on the table.
I know the balls on the table are old and worn down, but there is a noticeable difference in size between their measle balls and the balls provided with the table.
When I show them my cue ball, a red triangle, is the same size as the other balls, they don't want to use it.
To me, I think the measle ball reacts differently than my ball. I can move the measle ball a lot easier than my ball and so can they. It takes more of a stroke to draw my cue ball than it does the measle ball and they don't like that. I have no problem moving it around.
I don't know if they have "real" measle balls or knockoffs, but they all claim they are the real deal and EVERY one of them are larger than the other balls on the table.