Here are the instructions from TechGlitz:
Step 1: Dial *3001#12345#* on your iPhone then press Call.
Step 2: You'll now enter Field Test mode. You'll notice a signal strength indicator on the upper left hand corner of your screen. You can toggle between signal strength bars and a number by simply taping on it. At this point, press the home button to exit the app or if you would like the feature to be permanent, continue on. Don't worry, the process is completely reversible.
Step 3: Hold down on the Power button until you see the ‘slide to power off' bar. Do not power off the device.
Step 4: Press down on the Home button until the app closes and you return to your home screen.
That's it. Even when restarting the iPhone, the signal strength can be toggled between bars and numbers. The closer your number is to zero, the stronger your signal is.The lower your number, the stronger your signal—but don't include the - sign when evaluating (so, if the signal says -60, that's better than -90).
This trick may come in handy when you're in a dead zone and need to know where to move to for a better signal. (If you're constantly suffering from poor signal woes)
By the way, Android users can take a look at their dBm number under Settings > About Phone (it may be under Network or Status, depending on your phone).