Apps that support Dynamic Type will adjust to user’s preferred reading size. They can change the font size on their iPhone, iPad, and iPod touch. Here are two options to quickly test for different text sizes on iOS.
Option 1: Text Size control in Control Center
Customize Control Center to include Text Size as control. If you also want to test for Larger Accessibility Text Sizes, make sure to enable Larger Accessibility Sizes in Settings > Accessibility > Display & Text Size > Larger Text. Text Size control will now show more options, imitating the stepped slider at the bottom of Larger Text settings. Now access Control Center before opening your app to set the text size you want to test with.
If your app observes didChangeNotification and responds when the value of the propertypreferredContentSizeCategory changes, you can also change the text size within the app. Make sure to enable Access Within Apps in Control Center settings.
Option 2: Shortcuts
Create a custom shortcut which sets the text size to either your preferred text size or to Ask Each Time if you want to choose every time when your shortcut runs. You can customize the details but make sure to give it a proper name.
In general, you can run the shortcut in various ways. Even faster is to use Back Tap (iPhone only) and assign the previous created shortcut to either Double Tap or Triple Tap. You can also create a second shortcut for a different text size and assign it to the other option to quickly move back and forth between these two sizes.
Personal Experience
Depending on how you actually use your device while testing, one option might be faster than the other. I found that using shortcuts via Double Tap and Triple Tap (one shortcut assigned to each) is faster in hand-held mode and does involve less actions — although the confirmation notification still bugs me. Laid down, changing the text size via Control Center is more efficient. In terms of changing the text size within the app, you may end up with a broken user interface whereas closing and re-opening the app is then the preferred way to properly test the application.