How do you know if someone is tapping your cell phone?

You don't.

Cell phone signals are transmitted by radio waves and there's no way to stop any one else with a receiver from listening in. They'd need a device that can keep the calls separate, but the technology is not complicated. The cell phone provider would not have to participate. They couldn't tell if anyone's listening either.

Encryption is available, but the person you are calling has to have the ability to decrypt, so for most of us, that's not practical.

Bottom line --- don't say or do anything on a cell phone that could get you in trouble.