This page takes you through estimating the heat output and the efficiency:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

Its an approximate method, but it should get you in the ball park.  You will need a way to measure the airflow velocity out of the exit vent.  The CFM ratings on the fan don't mean much because they go down as the flow resistance through the collector goes up.  There are some cheap ways to measure airflow here:
http://www.builditsolar.com/References/Measurements/measurments.htm

Basically, a solar air heating collector is going to be most efficient when it has enough airflow so that the temperature rise through the collector is just enough to be useful for heating -- this is usually in the 40F to 60F range.   As a general rule, if the temperature rise is more than this, the collector does not have enough airflow, and the absorber is running too hot and losing a lot of heat out the glazing.
Its good to have about 2 cfm per sqft of collector area.

You can have a temperature rise in the right area, and still have a not very efficient collector -- the main reason would be that the air is not flowing evenly over all of the absorber.  That is, there are dead air areas that are not getting enough airflow to carry the heat away.  Not sure how you find these dead air areas, but if you (or anyone) works out a good way please let me know :)  -- maybe smoke?

One more thing to look at:
http://www.builditsolar.com/Experimental/AirCollectors/AirCollectors.htm