Wiktionary
know which end is up
alt. (context idiomatic English) To possess sound judgment or common sense; to have a clear understanding of a situation. vb. (context idiomatic English) To possess sound judgment or common sense; to have a clear understanding of a situation.
Usage examples of "know which end is up".
Between the wedding and Santini's entourage and their fundraiser and Christmas, I don't know which end is up today.
I may sell Bibles but I know which end is up and I wasn't born yesterday and I know where I'm going!
A traffic jam when everybody is floating, and you don't know which end is up, is about eight times as confusing as an ordinary one.