Instead of 1 to 5 dots or bars get a real number for signal strength. This is the better way to test signal quality.

Accessing Field Test Mode on the iPhone is relatively simple, just open the Phone app, switch to the keypad and dial the following code: *3001#12345#* and then press call. If you dialed it correctly, your iPhone will enter Field Test Mode and you’ll see the numerical value for signal strength in the upper left hand corner of the screen where the signal strength was previously displayed in bars. To exit and return your iPhone to normal status, all you need to do is hit the Home button. The mode is available on any iPhone running iOS 4.1 and all later versions.

If you want your iPhone to always display numerical signal strength instead of signal bars, you can perform the following process. Once in Field-test mode (accessed by entering and dialing the code above), hold down the power button until you see “Slide to Power Off”, then release it. Then hold the Home button until you’re returned to your main app screen. You’ll now see your numerical signal strength while you use your phone, and you’ll be able to tap the signal numbers to switch to signal bars, and vice versa. To exit this permanent field-test mode, simply reboot the phone or re-load Field Test Mode and exit it via the Home button.