ORIGINAL: AirWizard
I would tape 2 pushrods on the elevators and have them pointed back and almost touching behind the rudder. Then go through the movements and see if the are working the same
Here is a bit more info on the method described by AirWizard,
Scroll down this page until you get
An Idiot Proof Elevator Differential Gauge on this page [link=http://www.rcpattern.nxs.net/tipsB.htm]http://www.rcpattern.nxs.net/tipsB.htm[/link].
Cheers
Pupmeister