1. random outburst of genuine childlike excitement/emotion in the middle of a seemingly normal conversation
2. bright eyes
3. ability to say things that would be offensive coming from anyone else and people love them more for it somehow
4. if they're attracted to you they will probably (but not always) be more reserved around you than other people
5. a lot of people probably won't appreciate their intelligence/ability to have a deep conversation but it's there and can appear even in crazy outgoing social gatherings
6. very natural flow between serious conversation and zanyness
7. seem to be flirting with everyone at times (remember #4)
The conversation, in my experience, is generally very natural and easy between INTJ and ENFP. Although, I've often seen the interaction between INTJ/ENFP end up with the INTJ being the more outgoing of the two.
I'm sure I could come up with more but that's it off of the top of my head.