Discriminate User-Login by Cookie

[Mrfire]

Hi all!

I would like to know how to know if the current user is logged in by cookie check.
i mean, as i dont' know very well how vB works about the login System (i've already read many post on this forum about...), i'm wondering if vB set a cookie on the client that make possible my site (on another server - another platform - another subdomain) can know if the current user is CURRENTLY LOGGED IN.

I know that vB set a cookie like "bbUserId" (and that i can set cookie readable from all subdomain) ... but i'm afraid that this cookie will be available ALSO when the forum sessions are EXPIRED, when the user close the page immediately after LogOut and before the php can clear the Cookies, When the users hack his cookie system or the Clock of the computer...

I'm thinking some alternative solutions, as: to constrain vBulletin to set a Cookie with the SessionID of the current user, so myExternalSystem can call (server-side) a little php page (i will build) that give it a response about the state of the session that have SessionID as passed in parameters, that's the SessionID myExtSys has read in the Cookies. if php return that these sessionID correspond to a CURRENTLY LOGGED USER, myExtSys can store the UsersInfo in myExtSys.Session and make accessible some section of my site.
When myExtSys.Session EXPIRES... i check again the cookie... and again all.

May this solution works? Is there an easier solution?
maybe the single 'bbUserID' cookie is already a SURE check that the User is CURRENTLY LOGGED IN?

have you any solution/suggest/link?

Thank you!