Park on a reasonable hill
Mower in neutral , brake off give it a push
A good hydro will be hard to push and stop moving when you stop pushing
A totally stuffed one will race away under it's own force the second you let the brake off .
I have a tilting trailer that I collect mowers on and that is my test .
There are a lot of other things that can cause your symptoms from excessively worn belt, either pulley worn ,stripped spline on the gearbox pulley .