How to check if SwiftUI previews are running

ExampleApp.swift

import SwiftUI

@main
struct ExampleApp: App {
    init() {
        if ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1" {
            // SwiftUI Preview is running.
            // Do nothing.
        } else {
            FirebaseApp.configure()
            let settings = FirestoreSettings()
            Firestore.firestore().settings = settings
        }
    }
  
  // ...
}