Remember, whose is possessive. That means that whose is normally followed by a noun. If the sentence has a noun immediately after the whose or who's, you should use whose. If there's no noun or an article, use who's.

Whose or who's example?

The formula: who + is, or who + has. For example: who's hungry? Whose is a possessive pronoun. Use it when you're asking (or telling) to whom something belongs.

WHEN TO USE whose vs Who's?

Whose is the possessive form of the pronoun who, while who's is a contraction of the words who is or who has. However, many people still find whose and who's particularly confusing because, in English, an apostrophe followed by an s usually indicates the possessive form of a word.