Sadly there is no way you can prove anything to the customer.
You can test,, I can tell you how to build a test panel, two methods, but thats all you can do.
here is how to test
You need to build a test panel the easiest test panel is a 50 amp RV type service box mounted on a piece of plywood,, You need a PIG TAIL type connector to feed it power,, You will need to expose the black or black and red wires (not bare the copper but expose the black/red insulation) so you can clamp on a clamp on ammerter or.. you can put an in-line ammeter (you need two) on the board.
Circuit breakers (20 amp) to 4 duplex recpticals on each side of the panel
One outlet per breaker
Plug in 4 Space heaters, 1500 watt (12 amp) that's 48 amps, Verify with meter
Move to other side,
That is the only way I know to test to insure you are getting full power
Oh, and measure voltage across the outlet, (240 volts across, 120 side to top or bottom, this assumes round pin at bottom)
