I've known Myers-Briggs for 17 years now and have "typewatched" for quite some time. Although individuals can differ, here is my general take.
NTs - I can spot them a mile away. They just have a very potent obvious quality that stands out like no other.
One NT type gives me problems though. ENTPs. They can appear to be so like ESTPs. And I haven't met a whole lot of ENTPs, that I know of, so I don't have a good feel for them. It took a year of reading ESTP/ENTP descriptions and analyzing my ENTP boss to death, before I decided on ENTP. Then I asked him to take the test, and indeed he was.
SPs - I can also spot these a mile away. No problemo. And can usually get all four letters right.
NFs - Most I can spot. But again, sometimes I can have problems between SF and NF. And INFP (my own type) are the worst. I have four INFP freinds (I know because they told me so) and my father is INFP, yet, I find an unknown INFP to be very difficult to spot. There were two guys at work, that I finally decided were INFP. If I meet someone who is very vague and obtuse and very hard to read, and quiet, I tend to think INFP, but leave my opinion open.
SJs - yeah, they seem pretty straight forward. I may not get the E/I or T/F, but can get the SJ pretty quick.
So, what about you? And what kind of clues do you look for when trying to decide a type? I'm talking about people irl. Or do you try to guess?