It's only "it's" if it is "it is."

If it is not "it is," it's "its."

-----

Or to be clear (lol),

1) The possessive of "it" is "its."

2) "It's" is a contraction of "it is."

> 2) "It's" is a contraction of "it is."

... or "it has".

--

EDIT: I find it much easier to remember that "its" is (only) the possesive pronoun.