You cant ... you can do it like ... by recognizing gear they have ...
example ... You see someone in 8th degree with 8th degree sword and shield and all parts of 8th degree armor except chest ...
meaning he is lv.68 ...
Because first tier of 8th degree armor starts at lv.64
64- hands + weapon + shield
65- shoulders
66- legs
67- head
68- legs
69- chest
Since he is missing chest part , he is lv.68 (or doesnt have cash to buy chest

) ...
That is the only way ... (or you can invite them to party to see what level they are) ...
I tought that recognizing their level by gear will be complicated ... but it really isnt ... You'll learn it from time to time ...