Older pheasants will have longer tail feathers and longer spurs. Also, a when you hold the rooster by the beak this years birds will bend, while a mature birds won't.
Pheasants are one of the most fun upland games I've hunted. I love to see the dogs get excited and start quartering back and forth when they pick up the scent. Then the rooster "explodes" out of the cover and is outa there in a hurry, if you're not on the ball. Wew! Exciting.