Why not just repot into a larger pot?

If it is truly root bound then, I agree that you should get a larger pot. But if you do keep it in the original pot, don't trim the roots. When transplanting, the goal with Hippeastrum is to keep as many as possible of those roots alive.

How do you tell when a bulb is actually root bound?

When there are more roots than soil and the roots are encircling the inside of the pot...fair bet it's root bound.

Or when the roots are coming out the drainage hole!!