This function uses the DSGetDCName API call to retrieve the name of a Domain Controller, site information etc. You can pass in the the NetBIOS name of a domain if you want to query a foreign doamin or if your client is in a workgroup, or just pass an empty string ("") for the default domain. 

Function DSGetDCName(const sTargetDomainName: String; out sDomainControllerName : string; out sDomainControllerAddress : string; out sDomainName : string; out sDnsForestName : string; out sClientSiteName : string): integer;
Type
    TDomainControllerInfoA = record
    DomainControllerName: LPSTR;
    DomainControllerAddress: LPSTR;
    DomainControllerAddressType: ULONG;
    DomainGuid: TGUID;
    DomainName: LPSTR;
    DnsForestName: LPSTR;
    Flags: ULONG;
    DcSiteName: LPSTR;
    ClientSiteName: LPSTR;
end;
PDomainControllerInfoA = ^TDomainControllerInfoA;
Type_DsGetDcName = function(ComputerName, DomainName: PChar; DomainGuid: PGUID; SiteName: PChar; Flags: ULONG; var DomainControllerInfo: PDomainControllerInfoA): longint; stdcall;

const
    DS_IS_FLAT_NAME = $00010000;
    DS_RETURN_DNS_NAME = $40000000;
var
    DomainControllerInfo: PDomainControllerInfoA;
    lngResultCode : LongInt;
    iResultCode : integer;
    _DsGetDcName : Type_DsGetDcName;
    pTargetDomain : PChar;

begin
    sDomainControllerName:='';
    sDomainControllerAddress:='';
    sDomainName:='';
    sDnsForestName:='';
    sClientSiteName:='';
    if sTargetDomainName = '' then
        pTargetDomain:=nil
    else
        pTargetDomain:=PChar(sTargetDomainName);
    try
        iResultCode:=LoadLibrary(pchar('NetAPI32.dll'));
        @_DsGetDcName:=GetProcAddress(iResultCode,pchar('DsGetDcNameA'));
        lngResultCode:=_DsGetDcName(nil,pTargetDomain , nil, nil, DS_IS_FLAT_NAME or     DS_RETURN_DNS_NAME, DomainControllerInfo);
        FreeLibrary(iResultCode);
        if lngResultCode = NO_ERROR then begin
            sDomainControllerName:=DomainControllerInfo^.DomainControllerName;
            sDomainControllerAddress:=DomainControllerInfo^.DomainControllerAddress;
            sDomainName:=DomainControllerInfo^.DomainName;
            sDnsForestName:=DomainControllerInfo^.DnsForestName;
            sClientSiteName := DomainControllerInfo^.ClientSiteName;
            Result := 0;
            FreeBuffer(DomainControllerInfo);
        end;
    finally
    end;
    Result:=lngResultCode;
end;