It takes a lot of practice and experience (from a lot of practice) to judge the opponent's energy state. Then, just when you think you're starting to get the hang of it, you'll run across the guy who is a master at disguising his energy level and using it to deceive and kill you. 
. Never give up learning and expanding your experience base. 
QFT. Depending on the plane and pilot, it can be "hidden". But, even after some time you can spot it and it's not hidden so much (part of it is just knowing they're loaded up and now YOU have to watch them closely for if/when they pull in for a shot.)
