I just don't see Jack giving Randall any signals. When Randall smiles at him at the table, Jack's eyes move away from him. (One thing that was a little odd, was when Jack asked LaShawn to dance & Randall's eyes darted quickly away from Jack to LaShawn. What was that all about?) Yes, Jack looks away from LaShawn when they're dancing, but was he really looking at Randall? When they were outside & Randall suggests they go to the cabin, Jack stared blankly ahead & I don't think he gave any indication that he was interested. I think the small talk was Jack's way of passing time & keeping the conversation on things other than the 2 of them. It was obvious that Randall was pursuing Jack. But I think mention of going to the cabin only made Jack think of & want Ennis more. Am I wrong? Huh? 
I suspect that Jack moving away his gaze is a mannerism that was born of some inherent shyness.
Why do I say this? Another instance in the movie where such a thing occurs is when he first meets Lureen at the bar. Jack clearly was interested in Lureen at that moment, because he was flattered at her attention during the winning rodeo bull ride (as indicated by the screenplay for the bull riding scene, if I recall correctly).
But when Lureen returned his gaze, Jack immediately moves his gaze and nervously pretends not to notice. When Lureen confronts him, he bites his lip nervously and is tongue-tied (awesome acting from Jake Gyllenhaal in this scene!)
I think we can extrapolate this to the scene with Randall. The contrast is that when Randall looks at Jack and smiles, Jack coolly turns his gaze away and continues the conversation without a break or sign of nervousness. He has learned a bit of subtlety over they years, even though he was still inherently abashed in such situations.
But I must admit that one counter-example to this in the movie appears to be the scene with the Rodeo clown at the bar. Here, Jack meets the clown's gaze with a humger that is unmistakable -- enough to unnerve the clown who makes a hasty retreat. Desperation on Jack's part, perhaps?