WHO’S: a contraction of “who is” OR “who has.”

WHOSE: a possessive of “who,” (i.e., to whom does this belong?)

The apostrophe in WHO’S is the main difficulty for people. Most of the time, an apostrophe -s indicates possession, but in this particular case, it is merely functioning as a contraction.

“Whose” is similar to “its,” another tricky pronoun. Remember, no possessive pronouns use an apostrophe -s.

For example, “WHO’S yelling in the house?!” (who is)
For example, “WHO’S already finished dinner?” (who has)

For example, “WHOSE stinky shoes are these?” (possessive of who)
For example,: “Do you know WHOSE candy bar this is?” (possessive of who)

TIP: Try replacing WHO’S/WHOSE with “who is” or “who has.” Does the meaning change? If no, you want who’s. If yes, you want whose.