You’re

You’re is a contraction of you are. It has no other uses. This is a 100% rule. If you cannot expand it to you are in your sentence, then it is wrong.

Examples:
The trouble with the rat race is that even if you win, you’re still a rat.
(Expands to you are – correct)
Doing nothing is very hard to do. You never know when you’re finished.
(Expands to you are – correct)
Please ask Joan to post the parcel when you’re in London.
(Expands to you are – correct)
You said that you couldn’t believe you’re ears.
(Does not expand to you are – should be your)

Your

The word your sits before another word (usually a noun or a pronoun) to show that it belongs to “you” (e.g., your car, your arm), is of “you” (e.g., your picture, your photograph) or is related to “you” (e.g., your uncle).

Your is a possessive adjective. (Other possessive adjectives are my, his, her, its, our and their.)

Examples:
Our expert will answer your questions about pensions and savings.
(questions belonging to “you”)
Sarah doesn’t look like your sister.
(related to “you”)