geoserver - How to find markers in selected dragbox openlayers 3?

Here is a part of code,

draw = new ol.interaction.DragBox({
            /* dragbox interaction is active only if alt key is pressed */

            condition: ol.events.condition.altKeyOnly,
            /* style the box */
            style: new ol.style.Style({
                stroke: new ol.style.Stroke({
                    color: [0, 0, 255, 1]
                })
            })
        });
        /* add the DragBox interaction to the map */
        map.addInteraction(draw);


        draw.on('boxend', function (evt) {}

I googled but didn't find how will I do. I created a dragbox and added to map , I have some markers on the map. How I learn that dragbox contains markers or not ? Thanks for responses.