This is a funny question that us current SOBO's have spent days talking about...we came up with a pretty comprehensive list of things that determine if someone is a thurhiker. I can't remember all of them (we talked about this is early NH) but I'll list a few i do recall.
1.the smell
2.facial hair
3.hair on their head is either long and or dirty and sticky looking
4.dirt all over the inside and back of the legs
5.footwear looks pretty crappy
6.salt lines on the pack/shots/shirt/etc.
7.they never know the names of things anymore without looking at their guide book
8.but.... they always know the distance between their start and finish
9.they pass by viewpoints
10.their trekking poles have duck tapped wrapped around the top
11.there is some amount of dryed blood on their body, that hasn't been cleaned off
12.they never know what day it is
those are some I could remember...and they keep us all laughing through our time in the whites