How to check if user with [UserName] and [Password] is domain administrator of [DomainName] without impersonation?
using (Impersonation im = new Impersonation(UserName, Domain, Password))
{
    System.Security.Principal.WindowsIdentity identity = System.Security.Principal.WindowsIdentity.GetCurrent();
    bool isDomainAdmin = identity.IsDomainAdmin(Domain, UserName, Password);
    if (!isDomainAdmin)
    {
        //deny access, for example
    }
}

public static bool IsDomainAdmin(this WindowsIdentity identity, string domain, string userName, string password)
{
    Domain d = Domain.GetDomain(new DirectoryContext(DirectoryContextType.Domain, domain, userName, password));

    using (DirectoryEntry de = d.GetDirectoryEntry())
    {
        byte[] domainSIdArray = (byte[])de.Properties["objectSid"].Value;
        SecurityIdentifier domainSId = new SecurityIdentifier(domainSIdArray, 0);
        SecurityIdentifier domainAdminsSId = new SecurityIdentifier(WellKnownSidType.AccountDomainAdminsSid, domainSId);
        WindowsPrincipal wp = new WindowsPrincipal(identity);
        return wp.IsInRole(domainAdminsSId);
    }
}

static void Main(string[] args) {
    string userDomain = "somedomain";
    string userName = "username";
    string password = "apassword";

    if (IsDomainAdmin(userDomain, userName)) {
        string fullUserName = userDomain + @"" + userName;
        PrincipalContext context = new PrincipalContext(
            ContextType.Domain, userDomain);
        if (context.ValidateCredentials(fullUserName, password)) {
            Console.WriteLine("Success!");
        }
    }
}

public static bool IsDomainAdmin(string domain, string userName) {
    string adminDn = GetAdminDn(domain);
    SearchResult result = (new DirectorySearcher(
        new DirectoryEntry("LDAP://" + domain),
        "(&(objectCategory=user)(samAccountName=" + userName + "))",
        new[] { "memberOf" })).FindOne();
    return result.Properties["memberOf"].Contains(adminDn);
}

public static string GetAdminDn(string domain) {
    return (string)(new DirectorySearcher(
        new DirectoryEntry("LDAP://" + domain),
        "(&(objectCategory=group)(cn=Domain Admins))")
        .FindOne().Properties["distinguishedname"][0]);
}

static string BuildOctetString(SecurityIdentifier sid)
{
    byte[] items = new byte[sid.BinaryLength];
    sid.GetBinaryForm(items, 0);
    StringBuilder sb = new StringBuilder();
    foreach (byte b in items)
    {
        sb.Append(b.ToString("X2"));
    }
    return sb.ToString();
}
public static bool IsDomainAdmin(string domain, string userName)
{
    using (DirectoryEntry domainEntry = new DirectoryEntry(string.Format("LDAP://{0}", domain)))
    {
        byte[] domainSIdArray = (byte[])domainEntry.Properties["objectSid"].Value;

        SecurityIdentifier domainSId = new SecurityIdentifier(domainSIdArray, 0);
        SecurityIdentifier domainAdminsSId = new SecurityIdentifier(WellKnownSidType.AccountDomainAdminsSid, domainSId);

        using (DirectoryEntry groupEntry = new DirectoryEntry(string.Format("LDAP://<SID={0}>", BuildOctetString(domainAdminsSId))))
        {
            string adminDn = groupEntry.Properties["distinguishedname"].Value as string;
            SearchResult result = (new DirectorySearcher(domainEntry, string.Format("(&(objectCategory=user)(samAccountName={0}))", userName), new[] { "memberOf" })).FindOne();
            return result.Properties["memberOf"].Contains(adminDn);
        }
    }
}