When someone is interested in me, I know almost immediately. I can just tell by the way they look at me, and the way they act. Their interest is genuine, and they kind of have a wide-eyed, curious, (maybe even deer-in-the-headlights) gaze. And if they're not admiring/showing interest quietly, it's rather obvious when they're flirting bluntly.
I'm really cocky with the people that i'm attracted to. I'll insult you playfully, or just have really witty comebacks. If you can be just as cocky and witty, you'll have me intrigued.
I think most ENFPs are really warm towards the people they like.. They might be openly affectionate, verbally and physically.
And if they make an effort to contact YOU and see YOU, that's a sign that they probably like you.. At least a little.