How can I tell if an aircraft (N-number) is already equipped with an ADS-B transponder?
If it's onesy twosy, you can try searching flight logs of specific aircraft to see if ADS-B output is recorded.  Here is an example: track log showing ADS-B returns

There is not a free database.  You could purchase a database query of flights that have been tracked by ADS-B.  Depending on the number of aircraft you need, this will affect the cost.  Flight Aware will quote this kind of job.

The FAA database does not provide this information.  It does provide mode-S code information, but that is whether or not the aircraft is so equipped - nothing for ADS-B (yet).