My experience is that the dominant intuition from ENTPs (and probably ENFPs, too) manifests itself very differently from the dominant thinking of INTPs. IMO it's probably better to distinguish between the two on this basis, since ENTPs can appear deceptively introverted.
If I may be allowed to speculate for a moment, it seems that INTPs possess their understanding of the universe in a massive, interconnecting web of ideas inside their head; and each new theory they come upon is integrated piece by piece in terms of logical relation to all their other understandings. When they have to solve a problem, they seem to first attempt to reduce the problem into something one of their theories can already explain -- if not, they get creative. But in any case, they appear to be much more willing to sift through the sands and pick through grain by grain than other types.
Extroverted intuition (for me, anyway) is far more chaotic and uncontrollable. My brain is always on, the gears spinning on overdrive, bombarding me with new ideas and thoughts -- and if I so much as try to shut it up, it just seems to keep working unconsciously. It's as if that idea-generating part of my brain has a mind of its own, and seems to work completely outside of my control. I often get ideas before I think of them, if that makes any sense. In fact, when I have to solve a problem it usually turns into a battle between me and my brain: there's me, frantically trying to sort through all the (sometimes nonsensical) ideas to follow-up on; and my brain, who's vomiting up volumes of these possible solutions and generating them in ways that are completely mysterious to me. The finely-tuned web of ideas that INTPs have is much less present for me.
I assume it's similar for ENFPs, too -- and probably INTJs, but the nature of Ni is a little beyond me.
Does that help at all?