Not so much a way of determining ENY, but a way to choose planes. I know most of us hate plane restrictions when they occur but, they're probably here to stay.
One thing that irks me is finding out, after I've chosen a plane, it's restricted. It gets even worse when the next plane you chose is restricted because the ENY changed just after you chose the first plane.
For instance; I prefer the Nikki ENY 5 or whatever. ENY says nothing under 8 so I pick the P51d. ENY changes to 10 right after I pick the P51D so then I have to go grab a 51B. Sometimes even after I pick the B model the restrictions have changed again and I'll end up in P38G.
I find that really annoying.
How about the hanger's plane list is color coded and have planes restricted under ENY a special color? That way you know what you can fly right off the bat and not get jerked around with frequent ENY changes. So, ENY is 10, all planes with ENY under 10 are highlighted in a certain color. Anything not that color you know is good to fly.
Red might be ok since it's already used when fighter hangars are down, but some might get confused taking a quick glance and think they can't fly anything from that field. Still, it might work.
Either that or perhaps blue could signify planes restricted under eny rules.
Last resort would be a permanent ENY box somewhere in the hangar that shows the current level of eny restriction. But, once you pick a plane under ENY, you should be good to at least get off the ground with it.