You access this feature from your Phone app, but before you do that you need to do a couple of things:
- Turn off Wi-Fi (Settings > WiFi)
- Turn off LTE, because these readings can be different from standard cellphone signals (Settings> Cellular>Enable LTE)
Now you've got that out of the way, go to your Phone app, switch to the keypad and dial the following:
Now press Call and you'll activate Field Test Mode. To exit, just press the Home button.
If you want to replace the signal strength dots with a decibel value, press and hold down the power button until the Slide to power off message appears. Then release the power button and then press and hold the Home button for around five to ten seconds until the Field Test Mode screen disappears.
If you want the dots back, simply switch your phone off and on (or enter the Field Test Mode and this time exit it by pressingHome).
I've been called a lot of things in my time, and "not a radio frequency engineer" is one of them, so if you want to know more about what all these settings mean you'll need to do some Googling/Wikipediaing.