A scale? It measures the total weight of the ball and enclosed air. Pressure may change, but the number of molecules inside the ball does not.
A scale could probably work — but, it would have to be quite precise. Air, at 70* F, and normal atmospheric pressure weighs 0.07492 lbs/ft^3, according to “The Engineering Toolbox”. Link here:
http://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html
You could probably use that site to work out how much less an under-inflated ball would weigh, compared to a standard one.
Just after I pressed “Post”, I thought of a couple of problems with using a weigh scale to test for compliance.
The ball weighs a lot more than the air inside of it, and they aren’t all exactly the same empty weight. You would have to have a precise weight for every empty ball — or for every ball that’s inflated to the right amount of pressure. It would be easier for game officials to just supervise the inflation of each ball.
Also, if you filled the ball with a heavier-than-air gas (CO2, for instance), there would be less pressure for the same weight of gas.
If someone wants to cheat badly enough, there are several ways to do that.