Where I hunt, the birds get a lot of pressure, so in my experience, I've found that the older birds tend to be the last in line. Obviously, it's not that way all the time, and every scenario is different, but I really do believe that you can read their body language to see which bird is the old resident. There are old birds that will walk away from a hen once he decides that she's not receptive to breeding. Meanwhile, many youngsters will dog them.

If you can see them with their strut let down, you can accurately compare body size/height. Obviously, you want a bird with a good beard too.