Send it to Sota.
They have an offer that they will free of charge test and confirm your equipment.
If you put a washer inside the coil, it should launch it like crazy. The inside of the coil the magnetic field is strongest. On the face of the coil, the outside, its not focused, and the washer will fly off to one side with a bit, not much, force.
The only way I could think to test it at home would be to get a compass, and see how far away it can move the compass. Then compare that to the Sota version.
I really don't trust cheap capacitors though. When they blow, its loud and dangerous.
A good capacitor isn't cheap.