I'd do the check on OnAppearing for each view:
var isAuthenticated = SecureStorage.Get("IsSignedin");
if (isAuthenticated) {
await this.routingService.NavigateTo("///MainPage");
} else {
await this.routingService.NavigateTo("///Signin");
}
Check out this page for more details: https://mallibone.com/post/xamarin-forms-shell-login