Launch the Phone app
Then tap on the Call button, which will launch the Field Test app (or enter the field test mode). You’ll also notice that the signal strength indicator has switched to a number from the traditional signal bar.
Now hold the Power button until the red slider (slide to power off button) appears.
Release the Power button and press the Home button until the Field Test app quits
The lower the number (in other words, the more negative) the worse the signal, and the higher the number (less negative) the better.
Anything above -80 is good, and would be considered full bars
Anything below -100 is bad, and would be considered few bars
For example, a signal number of -105 is considerably worse than a signal of -70. You’ll generally find that anything approaching -100 or lower is fairly bad reception, while anything above -80 is usually good, and if you tap the number signal it’s usually shown as full bars. The full range of the signal numbers extends from -40 to -120, with -120 being a nearly impossible number to see because it means no reception, and -40 would be about the strength you’d get being right alongside a cell tower.