Seriously though, INFP's just give off a certain wibe that makes them very easy for me to recognize. (Perhaps because i grew up with one) They are always calm and introverted in the gentle and kind way. They have that dreamy look in their eyes and usually show strong hints if individuality in their style and clothing, without going aggressively against the norm (Like an INTP would). So I'm usually able to type them by just seeing a picture of them.
Female ESFP's are very easy for me to type as well.
SPs. Or at least, I think the people I'm identifying are SPs. I'll recognize a male SP by their enthusiasm and forwardness - I sat near a boy on a train who I'm pretty sure was ESFP (though ESTP is possible). He was absolutely full of energy and spoke very forwardly - not in your face, but was very expressive. He had this constantly energised, might pounce on you at any moment presence, always moving his fingers. The main reason I pick ESFP over ESTP is because ESTPs usually have a Brian Blessed sort of over-the-topness in the way they speak. ISTPs have an intense focus, and what I'd call "an edgy tranquility", ISFPs I'm less sure on. If anything I've said here doesn't match up with your experiences, please inform me. Also, I'm less sure with identifying female SPs.
The most obvious, no-questions-asked types I've ever encountered are an ENTJ teacher I had, my ISTP grandfather, my ESFJ aunt, and a few INFP and ESTP acquaitances. Most other people I've met have required at least a bit more thought, because they don't fit the profiles to the point of self-parody like these people do. In general, I find it easier to type extroverts, because they are more likely to wear it on their sleeves. You don't have to get to know them as well to figure it out.
I'd say ESTP, INFP, and ENTJ are all incredibly easy to type to the dominance of their dominant functions. From there, it's pretty easy to determine the difference between ESTP from ESFP, INFP from ISFP, and ENTJ from ESTJ. ENTJ's are more rare than ESTJ's I'd say. I honestly can't think of a person in my life who is ENTJ besides me.