Simple function to check if the client has retina support.

retinacheck.js

function __isRetinaDisplay() {
    if (window.matchMedia) {
        var mq = window.matchMedia("only screen and (min--moz-device-pixel-ratio: 1.3), only screen and (-o-min-device-pixel-ratio: 2.6/2), only screen and (-webkit-min-device-pixel-ratio: 1.3), only screen  and (min-device-pixel-ratio: 1.3), only screen and (min-resolution: 1.3dppx)");
        return (mq && mq.matches || (window.devicePixelRatio > 1)); 
    }
}

window.isRetina = __isRetinaDisplay();