I’ve seen this somewhere before…

HTML

<span id="smScreenIndicator"></span>

CSS

#smScreenIndicator { float: left; }
@media (min-width: 400px) { /* whatever width suits the project */
#smScreenIndicator { float: none; }
}

JQuery

function isMobile() {
if ( $('#smScreenIndicator').css('float') == 'left' ) return true;
else return false;
}