I've never known that anybody is interested in me in THAT way until it's happened, though I've always notices grounds for thinking that they probably are........one of them wore a low-cut top when I visited her, and she kept bending over.......that felt like a pretty strong hint, but I still felt I couldn't be sure......then she asked me to give her a massage, which removed any doubt. Mostly they've been more subtle than that. Usually they've just said something or other that suggests they're OK about being alone with me....it's usually a throwaway comment that I can ignore or take them up on, e.g. they might want to go for a walk, and I'm free to say "off you go then" or "I'll come with you if you like."
I think it's more in the amount of attention the "couple" is giving each other. Of course that also happens in ordinary life but then there's always a logical reason for it. It's likely to be something more if there's no real excuse for the attention except for sheer interest. Assuming it's not an established platonic friend such as a family member. It crosses the line when I notice we're talking mostly about each other, exchanging our outlooks on life, our loves, hates and dreams, and bringing each other to life. In other words I don't go the way of hints and flirty stuff, I just find myself getting close to whoever, just like developing any other friendship, only we seem a bit more special to each other than that.
I don't really see the sexual side of it as such a big step, though of course it is exactly that. But I don't see the boundary so sharply. True, when it happens you know that you've probably got yourself a relationship (I don't get involved with one-night stands), and it's kind of good to seal the deal like that (sexual rivals usually go back into the woodwork then ), but I'm much happier to just let nature take its course and work on the friendship. Occasionally I've gone and confused the lady concerned by seeming uninterested in taking things any further than platonic, and that's precipitated some very weird games, but really the fault there is in my failing to raise the issue. You can talk frankly about sex without spoling anything, though I know some people would rather die than just say how they feel. Some weren't satisfied unless I told them I wanted them even though they'd just said they didn't want me.