Aircraft operators must verify that an aircraft is validly registered before the aircraft can be operated. Per 14 CFR part 47, section 47.3, no person may operate an aircraft unless it has been registered by its owner. Pilots should review the registration certificate on the aircraft to ensure that it is valid. Evidence of valid registration is an unexpired Certificate of Registration kept onboard.
For aircraft that have recently changed ownership, but the new Certificate of Registration has not been issued yet, temporary authority to operate is allowed under 14 CFR part 47, section 47.31(c) with a copy of the AC Form 8050-1 that has been submitted to the Registry onboard the aircraft.
For more information on aircraft registration, see the FAA Aircraft Registration website:
https://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/register_aircraft