Field  test mode is available in iPhone 4s iso5, Which will allow easy and precisely test your cell signal reception quality. Your signal bars will be replaced with a number like -85. The closer to 0 you are the stronger the signal is. Ok open up you phone and goto the dial pad then type in the following sequence and press call

*3001#12345#*

Side note: this has to be done from the dial pad will not work from a stored contact.