How to access your Android phone's hidden "Testing" menu
All mobile phones have some secret codes that don't come with the manual, so it's no wonder that such codes work on Android smartphones. We'll now show you a code that reveals a hidden Android Testing menu ("testing" is literally the name of this menu). The code is *#*#4636#*#* - you'll just need to introduce it via your phone's dialer. Once the last character is introduced, the Testing menu appears out of nowhere, showing you four sub-menus: Phone information (IMEI number, current network, etc.), Battery information (including health, voltage, and temperature), Usage statistics (showing the apps that were last used on your smartphone, including the exact usage time), and Wi-Fi information (besides finding Wi-Fi information, and being able to disable or enable networks, you can run ping tests from here - though you can also do this from the Phone info sub-menu).
While the *#*#4636#*#* code works on various Android smartphones, you may discover that it can't be used on some devices. Anyway, be careful when accessing the hidden Testing menu. If you change something, make sure you know what you're doing. For example, if you turn off the phone's radio (this can be done from the Testing -> Phone information sub-menu), you'll no longer be able to use your device as a phone - for calls, text messages, or cellular data - unless you turn the radio on again.
P.S.: The screenshots included below were taken on a Nexus 5 running stock Android 5.1.1. Things may look different on other devices.