A previous question answered this for android: by checking for firebase.test.lab System.Settings.
But how to detect running inside the firebase test lab in an iOS app? I did not find anything similar for iOS in the test lab documentation.
The recommended way to check that you're running in Firebase Test Lab is to send your own environment variable when you initiate a test. In iOS, there isn't an equivalent to Android's System/Global/Secure settings.
You may either set up the environment variable in your scheme, or modify your .xctestrun file by hand.