bentgear - Right. probligo mentioned the difference in towing with one tip missing. That is a good indicator of effectiveness. With a glider tip devices that work should show up in glide L/D, not necessarily an easy thing to measure at model size.

Measuring the time to bank to 45 degrees would allow the differential lift to be estimated. With a roll stable airplane the differences in a horizontal circle diameter would also allow the differential lift to be determined.

High speed maneuvers don't really gain anything that good wings arn't doing fine anyway. For example, why increase roll rate if it is outrageous already? Loops can be wrapped around the axle as is. Good styled tips are neat though.

Stall are a good thing to check the effectiveness of a tip plate. A good experiment for low speed flight gains would be to have a tip device on one wing and nothing on the other. Get height to recover from a stall and 2 spins and the time to say !@#$% and whoops. Slow down and slowly add up elevator and see which wing breaks first.

Since the stall characteristics are something that are repeatable if the tip plate is doing something good it will show up in the direction of the pre stall roll and of the snap. The strength of the pre stall roll, aileron needed to counter the roll, and snap rotation will be a good indicator also.