From: https://examine.com/nutrition/how-much-protein-can-i-eat-in-...

That being said, since fecal losses of protein and short-chain peptides tend to smell incredibly bad one can use a 'sniff-test' after bowel movements to assess if protein is being lost in the feces and thus not taken up by either the intestines or the muscle.

Not pleasant, perhaps, but should be effective :-)