Given a session id through the url, how can I test if that user is currently logged in?

I have a situation where another server is going to ask my web site if a user is logged in. This is odd but here it is:
1) I post to the foreign server the logged in user's sessionid

2) the foreign server posts back to me giving me the same sessionId through the url.
I have to determine if that session is is still active / the user is still logged in. If the use is still logged in, I write some text to a page.
Step 2 has be completely dumbfounded.
Any help is so much appreciated,
Kiley

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