Get the cheap bike speedometer.
(Google search the board for "
http://www.dealextreme.com/details.dx/sku.2682")
A better search will tell how to set the computer to read RPM from the K/H reading.
Searching "Cateye mighty8 mity8 enduro8" may get some better hits too.
Those have certainly been sitting for a while. Run the motor on a 12V battery for an hour or 4 to get the bearing grease and brushes a little smoother.
Put a magnet on the motor shaft.
Connect a motor to a fully charged battery at 12.7V or so.
See what RPM the motor is turning. Add about 10% to get an idea of the cut in if used as a generator.
G-