Inspecting elements on an iPhone is crucial for testing and troubleshooting web and mobile UI issues. This can be achieved through various methods such as using Safari developer tools or Sauce Labs, both of which allow developers to view and edit a website's source code, including HTML, CSS, and JavaScript, to identify and debug issues. To inspect elements on an iPhone with Safari developer tools, one needs to set up their device and computer by enabling web inspector for the Safari app, turning on the developer menu on a Mac, connecting both devices via a USB cable, and then accessing the Web Inspector window, which displays all the elements that can be inspected. Similarly, using Sauce Labs requires logging in to an account, selecting the iOS device to test on, navigating to the hidden dock area, opening Safari, restarting the simulator, and accessing the Web Inspector window. Additionally, Apple's web inspector feature is available for testing, but it comes with limitations such as requiring multiple devices and maintaining them, which can be time-consuming and expensive.