הוא תמיד ידע לאן נושבת הרוח.

English Translation

He could always tell which direction the wind was blowing.

is this meant literally. Or is it an idiom?

Are you asking about the English, the Hebrew, or both? I’m not sure which is your first language, but in English, “to know which way the wind is blowing” can be used literally, or it can be used idiomatically to mean “to know the direction in which things are heading”. I’m not sure whether this is true in Hebrew as well.