How to determine the type (AD User vs. AD Group) of an account?

Question

I have a question about determining the type (User or Group) of a account name.
For example, I have two strings, say \"Adventure-works\\david\" and \"Adventure-works\\ad

Answer 1

Warning: In case of using DirectorySearcher the accepted answer might fail, since objectCategory it doesn't return consistent results.

Consider using objectClass instead:

SearchResult sr = ds.FindOne();
bool isUser = sr.Properties["objectClass"]?.Contains("user") == true;
// OR
bool isGroup = sr.Properties["objectClass"]?.Contains("group") == true;