Determining Thread Safety in Unit Tests

I am writing a multi threaded application and I am also trying to work out how to write suitable unit tests for it. I think that's probably another question on how best to do that.

One more question, I have a class like the below knowing its not thread safe and want to prove it in a unit test but cannot work out how to do it:

public class MyClass
{
    private List<string> MyList = new List<string>();

    public void Add(string Data)
    {
        MyList.Add(Data);  //This is not thread safe!!
    }
}