90% of the time, the bull moose is looking at you when you are sizing him up. Ears are up and alert. About 18-20" on a younger bull, and 20-22" on a mature one. Laid out flat they will go about 28-30". I have taken and guided for a lot of bulls, and don't think I've ever seen one wider than 11" at the eye, most around 9.5".
Anything with antlers 10-12" wider than its alert ears is definately a trophy canidate!
Look at its body size and confirmation. Make sure it is 4-6 years old, and it will score good. Must check for the double or better shovels up front to get the score.