How can I tell if an ldap account is a user/service account versus a security group using c#

If anyone has a solution to this I am desperate..

 

I am given a string and I have to be able to tell if the value is a user account or a security group.  Anyone have any idea how I can do this.

 

Currently I use the following code to tell me if its a valid account...but I dont have any idea how to tell if its a user account or not. 

 

SearchResult result;
DirectorySearcher search = new DirectorySearcher();
search.Filter = String.Format("(cn={0})", username);
search.PropertiesTo ...