Unit testing a void method

I understand that you can unit test a void method by checking for its effects. However, looking at the loadConfigFile method in this code:

    internal XmlDocument configData;

    public ConfigFile()
    {
        configData = new XmlDocument();
    }

    /// <summary>
    /// Load config file into memory
    /// </summary>
    /// <param name="filename">path and filename of config file</param>
    public void loadConfigFile(string filename) 
    {
        if(string.IsNullOrEmpty(filename))
            throw new System.ArgumentException("You must specify a filename");

        try 
        {
            configData.Load(filename);
        }
        catch(Exception ex)
        {
            throw new Exception("Config file could not be loaded",ex);
        }
    }

It loads a config file into a private field - which needs to be kept private so that developers do not modify the value directly. Instead the modification will be done through setConfigValue and getConfigValue methods (which I would assume need to be tested separately).

Given this, how would I test that the loadConfigFile actually worked? As I cant access the private configData field.