How do I distinguish between a veteran and a new player?

[Ungod]

(interpret this as you will, it's not as simple as AD)

[Muratus del Mur]

A veteran will always have a lot more perks, bells and whistles on his account. Maybe medals, or part of an ally, or gold avatar, or best of all, a custom tag. Active days is certainly an indicator, but after the initial 100 days, the difference is not just days, unless we talk about really old players with thousands of days.