ORIGINAL: JoeRE
The above chart is about how I judge it. One thing I can add is the most dominate gobblers have their wingtips rubbed down the most because they do the most strutting...as a general rule.
Any male turkey that struts will have it's tips rubbed down.
Weight plays into it as well, but spur length is the main way. That's not always accurate, but generally speaking that's the way to tell. A 1 year old bird NORMALLY won't have 1 1/2" spurs, where as a 3 year old bird would.