Yeah, I don't believe that the pilots have any way of knowing that someone is using a cell phone.

Recently there was testing done with cell phones in hospitals and the only effect on equipment was holding a cellphone right next to the equipment, and even this was detectable but did not cause any real problems.

For the cellphone to be a problem it would have to be right next to the equipment involved, as the effect dimishes according to the inverse square law.

Cellphones are fine on airplanes, except that the FAA says they aren't. They don't really cause any problems, this is a myth along with the "portable electronic devices" myth.