You can access the returned string by casting the result to OkNegotiatedContentResult<string> and accessing its Content property.
[TestMethod]
public void TestGet()
{
IHttpActionResult actionResult = controller.Get();
var contentResult = actionResult as OkNegotiatedContentResult<string>;
Assert.AreEqual("", contentResult.Content);
}
Example code from: http://www.asp.net/web-api/overview/testing-and-debugging/unit-testing-controllers-in-web-api
