You will have to access the manufacturers information or you can use a Prestons Guide that will give basic ( and I do mean basic) information as to the date of manufacture and rated eff for that model condensor. It will not show eff for condensors installed to old coils or the eff of an older unit installed to a TXV coil. Preston's guide doesnt show any information reguarding cond & evap coil matchups.
You can also try the AHRINET.ORG web site They are the organization that certifies the efficiency and capacity of a/c's, ht pumps and such. If you have the condensor, evap coil & furnace model #'s you can get a certified rating for that combination. We print that off for all our customers so they know they are getting what they paid for.
I haven't used it to find older equipment so I do not know if that is possible or not. After I finish this I will try it and let you know if it works
Older equipment is more affected by it's condition than it's original SEER. Clean coils/blower, proper charge, and good ductwork can have that older 10SEER unit actually getting it's full 10 SEER. However that usually isn't the case, most older units are full of dirt, incorrectly charged, and have poor ductwork...