You don't need to use a separate function call to determine the user's signed in state if you've already added the sign-in button. The sign-in button's callback is going to trigger either on sign-in, when the page loads, or any time that the user's signed-in status changes. The page load trigger (immediate mode), will also help to indicate if the user is a Google signed-in user or not.
See monitoring the user's signed in status, which shows the different status fields that you can check (Google signed in, app signed in, or signed out).