assertValue() method is private (i.e. not documented), but it doesn't mean you can't call it

Using api config you can load you form like this:
Code:
contactForm.load({
    success: function(form, action) {
        var states = action.result.states;
        var stateCombo = form.findField('state');
        stateCombo.getStore().loadData(states);
        stateCombo.assertValue();
    }
});