The tower uses a simple hyper sensitive flux meter which will be designed for every tower according to the span of its coverage plans and these meters are usually digitized because the intensities must be controlled according to the change in humidity and other strong yet natural electrostatic forces. Any android or blackberry working system will show the signal intensity it is receiving and is pretty portable and accurate depending on the type of outputs you might want from them.
not EM wave sensitive device can be used as the carrier waves secure the connectivity.
a very simple device such as a microprocessor and its 2g trans receiver can be used as they are legal hardware modules with specific IMEI codes.