I do know for certain that the conceirges can check to see how many SEs are on the flight. We found that very useful for a recent YYC-FRA flight that I stupidly booked on a Sunday (we would usually never try for an upgrade on that day!) where J=4 on check-in and we were still waitlisted. Turns out we were the only SE in Y so our chances were good and we did get the last 2 seats after much sweating (2 may have been sold/used for J misconnects).

So that could be part of your strategy...(ie. if 5 SE trying to eupgrade and J=4 no point to try) but otherwise I would just suggest you keep winging it. If you think there is a slight chance of an upgrade go up to your "victim", introduce yourself and maybe show your name tag to try and make it look very legit, and tell them "I MIGHT be able to upgrade you on this flight with my extra upgrade credits and I like doing this for random people, could I have your name to give to the GA to try for the upgrade? I can't guarantee anything though..." If they look blankly at you move on... Good on you for doing this!