In my experience, when people are interested in you (whether male or female) they try to start a convo with you, coax you to start one, or work hard to keep the convo going. They'll smile and look warm and inviting. If you enter a room, their mood will often lift. They usually find some reason to keep you in their sights and may glance at you often. In convos, people who are interested in you will withhold personal opinions that they think are contrary to yours; they'll try to find out what interests you and keep the convo there, to entice you to talk more. When not interested, people usually respond with their personal thoughts regardless of what affect they'll have on you.
People who like you will start new conversations out of the blue based on your interests that they remember...they'll restart conversations you had before that they feel you 'bonded' a bit over. Or there might be teasing and light jabs (but these are really just opportunities for them to show you how well they know the 'real you' and what your limits are.) And if you're too busy to talk, they may feel and act slighted.
And people who like you will find opportunities for you to spend time together. They may suggest doing stuff after work or want to discuss hobbies of yours that (they say) they share.
Simply put, people who are interested in you will show that interest. People who aren't will likely just get on with their day...they may be very courteous, but they better respect personal boundaries and are less likely to risk intruding.
My two bits, anyway.
I think your idea sounds great, but to me it sounds like something that came right out of relationship book LOL.
Let's face it, most gay guys aren't working THAT hard to get someone's attention. In addition, just because someone does all those things does not mean there is a spark, which is another thing that a lot of us tend to mistake. The person could just be friendly and wanting a friend. Yet so and so over here is construing that to mean so and so likes them. But you don't like the guy who is showing all this interest because you feel he may like you, which you don't. So what do you do? Make every excuse in the book not to hangout.
But you are right on if the guy starts the conversation, there is some sparks to be had. If they didn't talk to you first, they didn't look at you first which means they didn't notice you first. Of course when there's 2 guys involved, it gets a bit harder to figure who's going to make the first move...so each one should showing the same level of interest.
I've ran into enough guys on adam4adam who seen me at the gym but who couldn't work up half the nerve to initiate any sort of vibe that would let me know they existed. BUT, they seen me the other day at such and such and thought I was hot...