Public ADS-B Performance Report Request Flight Standards provides this automated tool in order to assist aircraft owners, operators, and avionics shops with the validation of the performance of the ADS-B Out equipment installed on aircraft.
I know that transponder/encoders are required to be checked every two years but I believe the PAPR report is sufficient for the ADSB. From the FAA website:
https://adsbperformance.faa.gov/PAPRRequest.aspx
Actually many folks check them quite often. It's easy, free, and the results are sent to you within a few minutes. When I do mine I print a copy and put it in the aircraft records.