When are you in police custody?

You are in police custody after you have been informed that you are placed under arrest. Until you have been informed that you have been placed under arrest, you have the right to leave whenever you wish.

For Miranda rights, you are considered in police custody (and therefore Miranda applies to you) when you reasonably believe that you are being detained without the right to leave. Because of this all police have a procedure that requires the officer questioning you to tell you that you are being questioned and you are free to leave