One could know another. — Literally, a man could recognise his friend; i.e., before daylight, in the early dusk.
A woman. — Literally, the woman — i.e., this woman. Thus it is of Ruth, not of himself, that Boaz is here thinking. A sensible man like Boaz knows “that we must not only keep a good conscience, but keep a good name; we must avoid not only sin but scandal.” (Henry.)