knockoutjs determine if the checkbox is checked on click event

I have this checkbox control:

<input type="checkbox" data-bind="attr: { id: 'prefix_12', attributeid: 12, poseId: $parent.Id }, click: $root.addEffect" />
                                            Add Option

And the event code:

self.addEffect = function (c, event) {
    var target = event.target;
}

What I want to know is how can I determine if the checkbox is checked or unchecked for each click event?

Thanks