Active Directory Single Sign On

Question

I have figured out how to authenticate a user against Active Directory but now I need to know how to tell if a user has already been authenticated.  Here is the scenario that I am trying to accomplish:
A user comes into work and logs into their computer at work.  When they go to the administrative web site they are not prompted for their credentials again.  If the users goes to the administrative web site from their computer at home then they are asked for their credentials.  It seems pretty straight forward and I would suspect that developers have done it before but I cannot seem to find any information about it.  If someone could point me in the right direction that would be great.