How to verify Google if the user is logged in?

[Jin Fin]

I just developed an application (Jave with GAE) at appspot.com. It allows Google account to authenticate. However, when I sign off from Google account, my GAE cookie (ACSID, SACSID) doesn't synchronize with Google account and it still run as a valid user after the Google account logged out. If I want implement something when the user logout Google account, it will also force my app to logout as well. Is there any Google API for me to refer at?