Learn how the eight functions work, then watch to see which functions a person seems to most strongly demonstrate. From there it's a matter of narrowing it down.
If you can find two strong functions, which seems more prominent? If you'e only sure of one (say, Ne), then which are the two types that lead with that function? (ENTP or ENFP?) Which are the two that use it as an auxiliary? (INTP or INFP?)
I find it most easy to recognize S or N, on the basis of whether a person follows me or gives me a blank look. I can often tell Ne from Ni from whether a person cares to externalize and explore his notions, or just shrugs and says whether he agrees or not. Se is concerned with appreciating things, and Si is concerned with making sure everything fits in the correct slot.
Judgment is harder; you have to listen more closely to the precise conclusions that people come to. If you hear some strange leaps in logic, or judgments based on subjective values, that's probably some kind of an F. More of an Fe in the first case; more of an Fi in the latter.
Ti is easier for me to recognize, as it's my dominant function; I notice when people spend most of their cycles compulsively working through problems, to no particular end. Te is all about solving problems once and for all, then moving on. What you really need to do is this, they'll say. Go do it now! (Well, maybe I do, maybe I don't. Maybe I don't want to solve this just yet. Hang on a minute.)
Eventually you start to develop these archetypes in your head. Oh yes, you'll say. This person reminds me of X, Y, and Z other people I've known. She's probably of the same, or a similar, type. Then you can work from there to confirm.