How do you determine SEER rating?

The SEER rating is the Btu of cooling output during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. As you can see by definition one would have to operate a unit for an entire season and keep meticulous i(if not impossible) records to figure SEER oneself. The more practical method would be to read the literature associated with the unit you are intersted in. If you are trying to figure out what a unit you already have is, do a google search on the condensers mfr, and model # you may get lucky. If the unit is > 10-15 years old figure you are at 10 SEER or less.