Two issues usually determine that: 1. do you have a profit motive in calling it rent and 2. are you charging fair market rent (or more).
As a practical matter, If after deducting all allowable expenses, you are showing a profit, you probably have a rental situation. If you are showing a loss, you don't.
You may be calling it rent, others would call it helping with expenses by paying a fixed amount (as opposed to dividing up the bills each month).
