c# verify session

[Grunge]

Hia

We have a web based plattform where a teacher can reset the windows 
password of his students.
Workflow is like this:
Auth->Page->Studentdetails->ResetPassword --->/Windows C# 
WebServices->AD->setPassword./

So the Password will be resetted by a c# Services that runs on Windows.
How can this Service check if a passed Shib_Session_ID is valid and not 
older than 2minutes?
If anbody has a c# sample, class or lib for this, that would help me a lot.

Greetings & thanks
Philipp