I'd imagine it would be by the skills they have and the armor they're wearing. The more I think about the 'no levels' thing, the more excited I get. If anyone can do it right, it's SE. And if it's innovative enough, it could make having a character be about more than just numbers and stats.
edit - maybe you won't be able to tell at all. Maybe if the player doesn't advertise it, no one will know how strong he is.
Edited, Jun 25th 2009 11:19am by Kharmageddon
____________________________Future FFXIV Player
Anguish - 80 Death Knight (Retired)
Vor - 60 Warlock (pre-BC) (Retired)