Also in both the book and film in TS1 it is Ennis who takes control.It is not as if he is the passive one.Ditto the reunion scene.
However reluctant Ennis may be to accept the truth,he knows it somewhere,very deep inside and Jack recognises that almost from the word go.
I love the way you worded that, Fiona. This is a great and very interesting thread. My personal belief is that the "gaydar", so to speak, went off, like Fiona said, "from the word go". Jack gets out of his truck, kicks it, turns, sees the glory of Ennis, takes two steps forward as if he's going to introduce himself, Ennis lowers his hat/head, shielding his eyes from expressing/revealing any interest whatsoever, and then the camera goes back to a close-up of Jack's face, and there's a look that comes over him, when he sort of nods his head just a little, blinks his eyes knowingly. From the second time, and every one of the 30 times since then, that I've watched the film, that 4 or 5 second look that he's actually giving to himself, cause Ennis isn't looking -- that look, to me, meant "Ahhh, I see, you're not ready to admit to yourself who you are yet. Okay, I get it, fine, I can play that game." I don't know, those were the exact words that have always come to my mind that Jack was thinking to himself when he first laid his eyes on Ennis. And then he returns to the truck and poses himself there, just ready, able, and willing to start this game, at whatever cost.
Even though I think the interest was there from Ennis's perspective from the start, from the sideways glances in the trailer and the borrowing of the lighter and the revealing of the personal history at the bar, I think he dismissed the possibility as outrageous and improbable, if not impossible. But he kept it in the back of his mind that day/night. (Sidebar: wonder where they spent the night that first night???) The first time the average viewer gets to actually be privy to any admiration whatsoever between the two is when they're loading up the horses the next morning, and Ennis warns Jack that the horse he's on has a low startle point, and Jack, cockily (is that a word? if not, it should be.), states that he doubts there's a mare that could throw him, then we see Ennis's face, smiling to himself. To me, that look meant "man, this is gonna be fun...".
So, those are the two moments IMHO when I think Jack's and Ennis's respective "gaydars" went off about the other.