Just my subjective observations here, and I apologize for this text having no structure at all.
Knowing the function orders is a must, and knowing "how the functions think" is a must. No single clue is enough to tell someones type, look for several, the more the better.
General softness in someones voice or the way he/she looks at someone else may mean the person is more likely to be an F, lack of emotion or might mean T. Always context dependant, T's might be emotional and so on. Just generally speaking though, ESFJ's smile more than ISTJ's when they talk to people, you can't deny that. Fe expects reactions, like a thank you after being handed coffee. Fi is more contained, Fi users have a non threathening dreamy look to themselves, sometimes, but more often than other types, although Ni has it's own dreamy look, or then it's just INFJ's, or a particular INFJ, oh god I'm so lonely. Te does things, expects things done, Ti thinks "If things would be done, they would be done."
Look for how often a person suddenly "looks into emptiness" (the moment when people think about something or remember things), length is important too. Sensors I've found look less often and for longer, Intuitives more often and for a shorter periods of time, Ne and Se eyes do it in a more "bubbly" fashion and Ni and Si eyes seem to look "deeper". Never trust eyes alone, NEVER, but they sometimes give out so much about N and S. I've noticed sensors use more proverbs, (might be culture spesific, I'm from Finland) and intuitives tend to use them as a joke or something similar.
Listen to the words people use and what they say. Intuitives leave more blanks, they say alot with what is left unsaid. Sensors are more prone to wanting to know what happened where with who involved. Intuitives seem to generally speak more quickly, individual differencies everywhere though.
Look for functions other than the dominant, or functions acting through or with the help of the dominant. ISTJ's Ne isn't really different from ENFP Ne, both are whacky but the ISTJ tends to use Ne for the things and ideas he acquires through the Si Te filter, opposed to ENFP Ne applying for about everything in life it being the first function.
Watch the persons posture, how long he looks other people in the eyes, where he positions himself in a room, does the person open up straight away when approached or seem contained, does his general behaviour seem bubbly, careless, anything. Try to attach anything anyone does to a function, or think "what function would've caused that". Everything gives away something, hunches are good, wrong more often than I'd like, but hunches are good.