Everything you need to do this can be found in the DirectoryServices
namespace...

Bear in mind, though, that the currently authenticated user isn't
necessarily the account that ASP.NET is running under (in fact, it almost
certainly isn't).

This article shows you how to set file permissions:
http://www.codeproject.com/dotnet/adduseracetofile.asp?df=100&forumid=15281&exp=
0&select=821739
but the underlying process for checking permissions is virtually identical.

If you need any further assistance, I'd suggest you post in
microsoft.public.adsi.general, which is the newsgroup specifically for
ActiveDirectory matters. Make sure you tell them that you want to do this in
ASP.NET... Anything Joe Kaplan tells you can be considered as gospel...