string path = GetiPhoneDocumentsPath();
if (System.IO.File.Exists(path) == false) XmlTextWriter textWriter = new XmlTextWriter(path,null);

public static string GetiPhoneDocumentsPath()
{
    // Your game has read+write access to /var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/Documents
    // Application.dataPath returns
    // /var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/myappname.app/Data
    // Strip "/Data" from path
    string path = Application.dataPath.Substring(0, Application.dataPath.Length - 5);
    // Strip application name
    path = path.Substring(0, path.LastIndexOf('/'));
    return path + "/Documents/myFile.xml";
}

FileInfo info = new FileInfo(path);
    if (info == null || info.Exists == false) { ... };