Haha, it's kind of nerdy but I can relate. I wish I could just give everyone the test.
I actually have guessed a few correctly, I put a lot of thought into it. I think I'm especially good an picking INFPs out because they are much like me but more sensitive, or *dreamy* types, if you will. More idealistic than me.
I can usually figure out when someone else is an NT because we have the same sort of intellect and question everything around us. INTP/J are usually obvious to me, we have the same insight and slightly bizarre/unconventional views, while being very logical and somewhat reclusive.
Otherwise, it can be hard to tell. I can usually tell when someone uses sensing over intuition because they just seem to be more, "with the program", not removed from their surroundings at all.
If someone is extremely passionate at the expense of using logic they are usually a feeler, but it's hard to tell as the functions present themselves differently depending on the situation and of course have varying degrees.
I think if I knew someone pretty well or if they were like in some ways I could type them bu otherwise it can be very difficult and surprising.