Check user switched?

Yep! There is a function called current_user_switched() in the plugin. It returns false if the user isn’t logged in or they didn’t switch in. It returns their old WP_User object (which evaluates to true) if the user switched into the current user account.

Example:

$old = current_user_switched();

if ( $old ) {
  printf( 'Your old username is %s', $old->user_login );
} else {
  echo 'You did not switch in to your account';
}