Radomes are stripped and inspected before repainting. Scheduled maintenance includes using a test meter to check for moisture ingress. Moisture is plotted and for minor defects, the contaminated area repaired before repainting and return to service. In the event of major repairs, radomes are sent out to specialist centres that can perform attenuation testing after the repairs are completed.
Radar equipment is calibrated on the bench in accordance with the overhaul manual, prior to certification and return to service.
The Weather Radar equipment, like all other equipment installed in a commercial transport aeroplane, is built to a standard (TSO or Technical Standard Order) and certified to meet those specific measurable standards. Whether it is brand new from the manufacturer, or returning to service after repair or overhaul, all equipment is tested against the certified design standard prior to release to service.
(The above comments apply to commercial air transport operations under an AOC, that must comply with an Approved Maintenance Programme. I can't speak directly for General Aviation, but their requirements are similar.)