From FB PHP SKD
How to know a user switch the their account to use as page account when the user switched "Use Facebook as PAGE_NAME"?
Many thanks for any your advice.
User cannot interact with your application canvas and application tab canvas while working in a "Use as Page" mode. He will be prompted to switch back to be a regular user before visiting canvas.
Update:
Just checked and is the same for Sites using Facebook JS-SDK. While old cookie set by JS-SDK is preserved and not removed and no auth related events is fired using FB.login prompts user to switch to his normal account.