Except if you already have that biological variable measured, with a test that do not ask for effort. Then for each candidate, you can measure how they will per

Except if you already have that biological variable measured, with a test that do not ask for effort. Then for each candidate, you can measure how they will perform, compared to their expected performance, depending on their lung capacity or grip strength. Reaching their limit (or going beyond them) should be a good indicator of 'grit', since reaching one's physical limits is usually painfull and/or difficult.