Press the Home button to launch the phone's home screen. As you know, the dots on the top left corner of the screen indicate signal strength.
Tap on Phone > Keypad > Dial *3001#12345#* and tap on Call.
As soon as you tap the Call button, the Field Test app will open. You’ll notice that instead of the dots, you’ve got a negative number in its place.
While in the Field Test app, hold down the 'Sleep/Turn Off' button until the 'Slide to Power Off' screen shows up.
Now, instead of turning the phone off, release the 'Sleep/Turn Off' button and hold the 'Home' button for a couple of seconds till the Field Test exits and it returns you to the Home Screen.
There you have it! The signal will be displayed as a number. In my case it is -83dBm.
Reading the display: -50dBm to -60dBm = Strongest signal strength. Greater than -95dBm = Satisfactory signal. Less than -110dBm = Call-dropping bad signal reception.
To switch back to bars/dots, repeat Steps 2 - 4. Now, tap on the numeric signal strength to change it back to bars/dots. Then press the Home button to exit Field Test mode, and that's that!