Detection of the 3G network
You might consider the RTL-SDR software, which typically uses an inexpensive tuner based on the RTL2832U chipset.  You'd want to select a dongle/hardware that contains the frequency range of interest (for GSM, the Elonics E4000 would likely do alright).  Consider the legality of scanning in your country, though.  Note that in the US, "it is a federal crime to monitor cellular phone calls." 

A basic build guide for a scanner built using this approach can be found on Adafruit.  The general steps are:


Get a dongle supported by RTL-SDR
Install the Osmocom RTL-SDR libraries
Try starting out with the GQRX software; which is the recommended starting software from the RTL-SDR site for Linux (unsure of your OS, but the link contains a Windows guide too).


The "RTLSDR Scanner" tool  appears useful to analyze the spectral results from the scanner hardware (e.g. this is a software spectrum analyzer).

See also: Decoding unencrypted GSM traffic using RTL-SDR