[ he resists the urge to nod — but even that wouldn't be really true. mamoru's not ordinary himself, in spite of being the only "senshi" who was originally human, and he has an innate ability to discern what's right from what's wrong, whether it be a feeling he has or something that (literally) came to him in a dream. ]
I suppose it would be kind of hard to tell whether someone's human or not if humans and aliens all looked the same.