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Depending on how you are handling sign in, a stored or even cloud variable would be the best way to check if a user has already signed up or to check to see if the user previously authenticated.
When a user opens your app for the very first time and signs up or signs in, you can create a stored or cloud variable and use a boolean (true/false) to set the variable. Each time the user opens the app after that, you can check for the variable and if they have signed up, you can navigate them directly into the app vs have them go to the sign up/sign in screen.