Applies to iOS 7 to 10
First, set your iPhone to field test mode: dial
with the Phone app. The field test mode holds a lot of technical details and information, intersting probably for technicians only.
Second, press and hold the sleep button until the Slide to Power Off screen appears.
Third, press and hold the Home button until the Home screen is displayed again. (Don’t tap any other button!)
Instead of an array of little dots, you should now see a negative integer in the top left corner. This is the so called Received Signal Strength Indication (RSSI) measured in dB mW. That’s a logarithmic scale; the higher the number the better.
To toggle between dots and number display, just tap onto the upper left corner.