The captain everyone trusts is the one who runs every drill full speed, whether the coach is watching or not.

Full speed at a drill nobody is grading is the cheapest available proof of character. Everyone performs when the coach is watching, so that tells you nothing, and the gap between the two versions of a person is exactly what a team learns to measure. A captain is not chosen for the loudest speech. The armband goes to whoever is the same in both situations, because that is the person others can model themselves on.

Pick the drill you usually coast through and stop coasting, starting at the next session. Say nothing about it. And notice who else does it, because those are the teammates worth following.