Use CSS media queries and Javascript together to determine browser width.

Use JS and CSS media queries together to determine width of browser window - CSS

/* position static in mobile media queries by default */
#js-media-test { position: static; }

/* and position it relative in larger sizes 
   you can set the break to whatever makes the most sense */
@media (max-width: 769px) {
	#js-media-test { position: relative; }
}

Use JS and CSS media queries together to determine width of browser window - HTML

<!-- add this html to just before the closing body tag -->
<div id="js-media-test" class="hide"></div>

Use JS and CSS media queries together to determine width of browser window - JS

// Now you can test the css property much easier than testing
// screen width etc.
$(document).ready(function(){ 
	if ( $('#js-media-test').css("position") == "relative") {
		// do something
	} else {
		// do something else
	}
}

For a much cooler way of doing the same thing check this out from my friend Boots. https://gist.github.com/bootsified/47b79fbf12ff7eccef66