Take the upper off the Gen3 gun, and Gen4 gun, both with the same sights.
Have an assistant take one of the uppers and install on the Gen4 lower, load one round then hand it to you without telling you which it is. Have them record the sequence in which they hand it to you. Hand each to you in random fashion determined by a dice roll 1-3 Gen3, 4-6 gen4.
Fire it without looking at it (blindfolded is best), hand it back to the assistant, then write down which upper you think you used. Shoot a Sig or XD in between each test to clear your senses. Do this 50 times with one type of ammo.
Look at the accuracy of your determination of recoil. You have to be better than 60% correct to really be convinced.