Now you can put your iPhone in field-test mode, which will allow you to easily test your signal strength. You just need to follow these steps:
1. Upgrade your iPhone 4 - or any iPhone - to iOS 4.1 2. Dial *3001#12345#* into your phone
Done. The signal bars will be replaced with a number (like -85). The higher the number it is, the worse the signal it is. To get out of it, hit the home button.