How to test Controller Action that uses JSON string from Request.Form?

I have an Action that gets JSON data from Request.Form[0] and has calls into domain objects.

I am testing this method, but it seems impossible to set Request.Form.

I could extract the method to another that takes the string it returns, but that would just be a one line method and the Action would still be untested.

Is there a method to test this or another, more testable method to get the JSON data from a $.ajax() call?

It's possible to pass a strongly typed string parameter writing it into the method, by adding it as a parameter

public JsonResult ActionName(string paramName)

and including it in the data:

var dataVar = getDataVar();
$.ajax({
    url: '/Controller/ActionName'
    , type: 'post'
    , data: { paramName: dataVar }
    , dataType: 'json'

    , success: function (returnJSON) {
    }
    , error: function (XMLHttpRequest, textStatus, errorThrown) {
    //error handle in here
    }
});