The reception bars at the top corner of an iPhone generally tell you how good the signal is to make and receive calls. This only shows five levels so it's not the most accurate reflection of signal strength. Here's a simple trick to let you find out the exact level of reception you're getting on your iPhone.
iPhone picture from Shutterstock
First, you need to get your phone into Field Test Mode by dialing *3001#12345#* and pressing the call button. Once you do that, you'll notice your reception bar has now been converted to a negative number. The closer the number is to zero, the stronger the signal strength, so a range of between -40 and -77 means you have a decent five bar signal. If you get -102 to -120, it means you're getting about one bar of reception.
Tech Insider has created a quick video which guides you through the whole process:
[via Tech Insider]