yo
speaking as an INTJ who is roommates with an ENFP, and thinks that accordingly, he might know a thing or two about the type, I could say that it should be really easy to 'oust' an INTJ into the open by simply appealing to the extraverted thinking in conversation, i.e. ask a LOT of questions. Kind of as a general rule, make sure these questions aren't as personally related to the INTJ either, although they could be slightly (I know, that sounds like such a weird tactic, but trust me). If anything, stick to the more theoretical in ideas to appeal to this thinking conversation "spark" as I call it. This isn't to say that you can't get a wealth of F-style information via this same method, later on when you know the INTJ better. I put this in italics because I've been told it is connotative of emphasis and importance. As for opening leads, and not to slam the nerd label down so quickly but seriously, Science Fiction is the art of ideas, it is most definitely a safe haven for INTJs. Isaac Asimov is purportedly an INTJ, and it makes sense. Anything with a system too, is usually that mystical bauble that can tease an INTJ out of even the deepest introversion. That could be anything from soccer (proven in this thread!) to astronomy, to comic books.
And I mean, that's the easiest element of the search for an INTJ, is when you feel like you might sense that someone you're talking to might be one, just turn your "asking questions" knob up to full blast. Most ENFPs often have to deal with the fact that they "ask too many questions" or are too inquisitive and well, courageous in their understanding of people. With an INTJ, "too many questions" is often just the right amount. From my own personal experience living day-to-day with an ENFP, I can tell you that I find the constant range of possibilities and questions asked to be inspiring and invigorating, as well as being able to personally enjoy and understand more about the asker through their responses to the knowledge and ideas provided.
Good luck on UnicornQuest.