One could, in principle, test for hollowness non-destructively by measuring its angular momentum. But I’m not sure how precisely one could do so using equipment in a typical home workshop.
A long ramp and a stopwatch would do the trick.
Oh all right, I’ll give more details than that.
If an object of radius R rolls down a ramp inclined at an angle θ to the vertical, one can show that its acceleration along the ramp is
a = g*sin(θ)/(1 + β),
where β is a factor that depends on the weight distribution of the object. Specifically, for a solid sphere of uniform density, β = 2/5, and for a thin spherical shell, β = 2/3. If the sphere is of uniform density but has “thick” walls, the result is somewhere in between.
So if you release the ball from rest at the top of a ramp of length L, and it takes a time t to get to the end, then you can determine the acceleration via the kinematic equation
L = (1/2) * a * t2, or
a = 2 * L/t2.
From that, you can figure out the value of β, which will give you a clue as to its internal mass distribution. For the cognoscenti reading this, the factor β is related to the moment of inertia of the object by I = β M R2.
(Also, I hope that none of my students are reading this, because I sometimes give this problem on exams.)
Ooh, yeah, I should have thought of that! The best I was coming up with was to put the sphere on an axle with a spool, and winding a string around the spool with a weight hanging from it. But your way is much simpler.