The only ways to prove drag change...

Wind tunnel.

Speed runs though a radar trap.

A wind tunnel.. you can mount the aircraft to the platform which links to the scale measuring how much the wind is pushing the model backward. then turn the wnd speed up to a specific value. Change tips and do the wind again. You will KNOW the airplane was at the exact same attitude and the only change was the wingtips. The scale would give a definitive number to the change. (Who's got the money for the wind tunnel time?) Yo will als have a scale that is hooked up to the model that measures the lift generated.

The high speed runs... you have a less positive measure because wind might change a bit, altering the measured speed through the trap. If the top speed change is small, its going to be hard to prove anything, as the record setting speed models tend to have a difference in thier speed each run with no change to the model.

Low speed runs can verify increased lift... because the model will stall at a lowerr speed. (but again... wind may make this hard to quantify)

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I can easilly do the empirical test with one wingtip on one side... and a different one on the other.... and I can prove there is a change. And from the way the model reacts to different wingtips I can estimate what those changes mean for real world model performance.

Thats WHY I used the carved tips on the glider I mentioned earlier... I wanted to be faster then the kit designer... it worked.