To find the signal strength of your cellphone, put the phone in the field test mode. If you are using an iPhone, open up your phone app, input the code "*3001#12345#*" and hit the call button to measure the signal strength.
While in the field test mode, you cannot see bars at the top left-hand side of your mobile phone. A negative number replaces these bars instead. These negative numbers can range between -40 and -130. The lower the negative number gets, the better the signal your phone has. A phone is at its best signal strength when it reaches -40.