Ahh John seems to be on top of it so forget what I said except for my ad :)
AD isn't my strong suit but I beleave you should be able to use the Path poroperty of DirectoryEntry to see if it's a user CN=user should be present, CN=Distribution Groups ect.
I know it's again the rules to post ads but I love this article and it's on subject and on the code project so not an ad. I love this
briliant article:
Howto: (Almost) Everything In Active Directory via C#[
^]
Edit: Just realized, it could be misinterpreted as it me who wrote the article. I have not written the article the article has been written by
thund3rstruck[
^]