Testing before or afterwards dont suppose really matters does it, its still a fault
A better thing when quoting would be to inform the client of the price and then notify them that you have a hourly charge for finding faults on on completion of the new board
Ironically i changed a board this morning, never tested before hand and powered it up, from experience i had already pre determined that they maybe a borrowed neutral from landing light, and there certainly was, so lighting circuits went onto the same mcb, not conventional maybe but not really breaking any rules