I think that ENFPs have a wide variety of behaviors on this subject. And, I'm guessing you think that ENFPs hop into a new relationship as soon as they leave an old one? I don't think this is true for all ENFPs. We tend to be polarized: either lots of hopping around or sort of INFJ-y.... by this I mean very cautious about taking the plunge but once they're in, they are IN for the long haul and very loyal. I fall into this second category.
For example, I have NEVER been one to casually date or be wishy washy in my romantic feelings about someone. I'm either not interested or fully engrossed. And, back to my first comment, I am very, very, very picky. Most potential suitors bore me. I've gotta find a guy reallllly interesting to even dally with the idea of dating him.
But I think to the casual observer it might be hard to differentiate between my casual and serious interest in someone. And thus it might appear I flit around aimlessly chatting people up. Not so. I know exactly what I'm doing. I'm being kind and friendly to everyone around me... and watching and learning about them, too.
So how does the non-ENFP tell the difference? I think that ENFP Saltine said it so well in this post
when she said, "I personally bond through vulnerability..." That's how to know if an ENFP is realllllly interested in you. Does she/he bare her soul to you? Not just funny comments about her/his banal and absurd life, but really deep and personal stuff. <Sigh.> I take that risk with very few indeed.