If you're finding that you need to provide a different experience for the people using your app depending on whether or not Zoom is enabled, I'd challenge you to task yourself why this is, and how you can tweak the design to work better for all users.
There should be very few reasons that you need to present a different UI for low vision users. If your app contains text, you should scale the text based on the user's preferred font size:
https://developer.apple.com/videos/play/wwdc2017/245/If your app uses 3 finger gestures and conflicts with Zoom gestures, you can notify the user of this conflict using this API:
https://developer.apple.com/videos/play/wwdc2017/245/You can also move Zoom's focus to a specific control if needed using this API. If Zoom is not on, this will have no effect, so you don't have to worry about whether or not Zoom is on:
https://developer.apple.com/documentation/uikit/uiaccessibility/1624921-zoomfocuschangedIf you are able to provide more specific details about what parts of the user experience in your app are dependent on Zoom running, I might also be able to recommend some alternatives, but without more details I hope the above is helpful. It's very rare that you'd need to legitimately know that Zoom is on in order to design a good experience.
If you are trying to determine if zoom is on for analytics data reasons, I don't think we have any options for you from an API standpoint, as this information can unnecessarily reveal information about the user's device that could be sensitive to them.
