Ratinator has asked the fundamental question and until that is answered there won't be a proper solution.
It can't be to check for legality or compliance with the law; a field test can never deliver the precision needed for a test that would stand up to scrutiny in a court of law.
That leaves the matter of cheating.
Why would you want more power? Presumably to flatten the trajectory and to diminish the flight time so that the wind has less effect.
Have a look at Chairgun and see how many clicks less you will need at 55 yards going from say 12 even as far as 15fpe. Roughly, what you give for 50 yards at 12 fpe will be correct for 55. Not a lot of change.
Now look at the wind drift. Imagine a 10mph crosswind all the way from muzzle to 55 yards. Yes I know it's a simplistic example but it makes it easier to demonstrate the point. The drift at 12fpe will be something like 170mm. Now hike the power to 15fpe, and it makes a massive
difference, it cuts the drift to something over 150mm. Half a kill zone difference. But, if my estimation of the wind had been less precise, and it was blowing at only 9mph and not the 10 mph that I had imagined, that alone would make the same kind of difference.
So I don't buy the argument that someone whose gun is running at 796 and not 795 or below has committed such a heinous crime and stolen a huge competitive march on the opposition that they deserve to be disqualified.
If you're really worried about extra power giving an advantage, and just think how many old hands run their Pro-T's at about 770 because that is an ideal setting, then introduce bonus points, the lower the velocity you run your gun at then the more bonus points you pick up. Now that would sort out the men from the boys. It might even make 22 calibre viable in FT.