If the police detain you the police have to have some reason to actually detain you such as investigation of a crime or suspecting you of a crime or you committed a crime.
You can ask the police if you're being detained and if the police doesn't say you're being detained then you're free to go.
If the police can't decide why they are detaining you then you should ask for a supervisor because once I had a police officer tell me I was being detained but the police officer couldn't decide why they were detaining me.
The police have to have a reason or reasonable suspicion to detain you and if not they must let you go.