How can I get the reference to currently active modal form?

You can get a reference to the currently active form by using the static Form.ActiveForm property.

Edit: If no Form has the focus, Form.ActiveForm will return null.
One way to get around this is to use the Application.OpenForms collection and retrieve the last item, witch will be the active Form when it is displayed using ShowDialog:

// using Linq:
var lastOpenedForm = Application.OpenForms.Cast<Form>().Last()
// or (without Linq):
var lastOpenedForm = Application.OpenForms[Application.OpenForms.Count - 1]