To find out your signal strength you can call *3001#12345#* from the keypad in the Phone app. Normally this cool function is only used by technicians but now it is on your hand.

If it works, your signal strength bars should be automatically replaced with a negative number. You can also tap the signal bar to show the exact number.

The closer to zero the better is your signal. −100 is OK, > −50 is good, an > −10 is perfect.

When done press the Home button and return to home screen.