IIS ASP sessions

I realize this isn't an IIS forum, but someone must have figured out how to tell when all the IIS ASP sessions have expired after removing a web server from rotation, making it safe to do maintenance. I know there is Perfmon to view active ASP sessions, but it is not accurate because I know there are sessions running even after perfmon says the count has dropped to zero. I've done some testing and it looks like IIS is re-using some sessions even after they've closed as long as the cookie and original session number still exist, but doesn't report them all being open. I can't find anything in Google.

 

 

Any input would be appreciated.