I am searching for a solution to identify phones simply by the signals it emits. Imagine a signal receiver in a room, then I want to know which people are using phones nearby. I don't care what they are used for, just the fact that they are turned on.
I can't really use Wifi or Bluetooth for this since it can be turned off. 3G/GSM etc. can also be turned off, but I would like to hear if it is possible to "sniff" these signals and identity them. I have to identify the signals since there are a specific group of people whose phones are "allowed" to be turned on. Then I want to know if anyone else than those people have their phone turned on nearby.
How is this possible?