Cocoa Dev something like this should work..
const string Domain = "SomeServer.Foo.Bar.com";
const string Username = "someUserSamAccountName";
PrincipalContext principalContext = new PrincipalContext(ContextType.Domain, Domain);
UserPrincipal userPrincipal = UserPrincipal.FindByIdentity(principalContext, Username);
replace with your values and this should return if a user is found or not.. I work with AD on a daily basis and just tested with my account creds hope this helps if so change your Method to pass in string samAccount , string DomanName;
hard coded example that you can follow
const string Domain = "SLO1.Foo.Bar.biz:389";
const string Container = @"DC=Foo,DC=Bar,DC=biz";
const string Username = @"cocoadev";//replace with actual samAccountName
PrincipalContext principalContext = new PrincipalContext(ContextType.Domain, Domain, Container);
UserPrincipal userPrincipal = UserPrincipal.FindByIdentity(principalContext, username);