If it is made since Jan. 06, is is 13 SEER minimum, and if it is older, and was not an "upscale" model, it is probably 10 SEER. If it is older than 20 years, it is probably 8 SEER.
Why the question? If you are looking at something new, they will tell you the SEER. If it is old, then it's old and not efficient.