Air Conditioner Operating Temperature?

In order to determine if an air conditioner (15K BTU/13K BTU) is operating at intended efficiency, what would the temperature be when inserting a probe directly into the fan exhaust box? This is assuming the unit has been running for some period with fan on high.

The service literature, as I recall, indicates a design of 16-20 degrees differential as measured between the return air temperature and the discharge air temperature. (IE. 78 deg. in, 58 to 62 out) Bear in mind this does not give you any indication of "Efficiency. Efficiency would take power consumption into account.