Remember, Jack had worked for Aguire before, so he knew what kind of work was ahead of them, and I think HE was sizing Ennis up, not only thinking of whether he could do the work, but what kind of company he was gonna be.
Obviously, whoever Jack worked with before, didnt end up in the same tent as him, so I do doubt, whether there was any ulterior motives to the looks he was giving Ennis.