It's a quite powerful sequence and brilliant direction by Lee. In the scene mentioned above, you nailed the description and perception of what's happening. Until we learn that Aguirre is going to hire two men this season both Jack and Ennis (and us) think it's either him or me for this summer job which both need badly. so I can see both plays from both perspectives. We are just not shown Ennis checking out Jack after that initial glance. that Ennis is really doing more here than checking out the job competition. While I definitely recognize Jack's gaze, body movements and stance after exiting the truck as expressing much more than checking out the job competition, I am not sure, from either perspective of: 1) knowing the story or 2) Naivete.
Those early opening scenes when Ennis and Jack first meet are indeed subject to interpretation couched in the story's context and a viewer's experiences.