One of three ways to debug a problem iPhone ad is through Chrome’s “device mode” tab.
Open up any page in Chrome. Either CTRL-Click or Right-click on the page to open up the menu and choose “Inspect Element”. This will bring up the developer tools. Notice there is a mobile phone icon in the corner.
Click this icon to bring up “Device Mode”. There are a number of options on this screen, but two are especially important for our purposes. First, pick a device. This will emulate the screen size of your chosen device, so you can replicate ads at those dimensions. This is useful for testing responsive ads, for example.
Second, you can fake the User Agent string. Many scripts look at the user agent to figure out what kind of device you’re using and faking it can be incredibly useful. If you’re trying to debug an ad that is targeted iPhone-only in the advertisers’ system, you could press refresh all day on your desktop and never see it. The advertiser looks at your user agent, sees that you’re not on mobile, and doesn’t show the misbehaving ad. By faking the user agent, you can trick them into showing it. Nice!
The “device mode” tab has LOTS of other options. For more details and an introduction video, see the official documentation: https://developer.chrome.com/devtools/docs/device-mode
Sometimes you need more control. In future articles, we’ll look in detail at two more advanced methods. First, you can connect your phone via USB and use Safari’s “inspect element”. The advantage here is that you’re using the real device, not emulation. In extreme cases, this still isn’t enough. Consider an ad that redirects the page so fast you don’t even have a chance to use “inspect element”. In these situations, you can pull a move out of the network engineer toolkit and setup a proxy to capture all traffic and inspect it later. Hopefully you never have to deal with this, but it does happen sometimes. Check out TechCrunch and a detailed writeup from Sentrant Labs for details.