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
AHRI or the manufacturers catalogue is where you should look to find SEER / EER / HSPF, etc... Even if a sticker is present, it could very possibly be wrong.
FYI: That the AHRI is very difficult to use. All you need for the database is a condensor (outdoor unit) model number, evaporator (indoor unit) model number, and if present, the furnace model number. Don't worry about filling an any of the other boxes. Also, LESS information is better. Start with just a few characters and keep adding until the database doesn't return any results. Back off the additional characters and then search manually. The key is that there are often substitutions and generic characters inserted into the AHRI entries that will throw off results (ie return no results) if you try to enter the entire model number for particular unit / combination.
RESNET has a list of age based efficiency defaults listed in their standards if the unit is really not listed.
Hope this helps. Good luck!
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...