var checkViewport = function(el){

	var minTop = jQuery(document).scrollTop(),
		minLeft = jQuery(document).scrollLeft(),

		maxTop = minTop + jQuery(window).height(),
		maxLeft = minLeft + jQuery(window).width(),

		$myElement = el,

		elementOffset = $myElement.offset(),
		elementHeight = $myElement.height(),
		elementWidth = $myElement.width();

	if (
		(elementOffset.top > minTop && elementOffset.top + elementHeight < maxTop) &&
		(elementOffset.left > minLeft && elementOffset.left + elementWidth < maxLeft)
	) {
		console.log('entire element is visible');
	} else {
		console.log('entire element is not visible');
	}
}

Copied from somewhere, probably stackoverflow.

Check if element is in the wieport

JavaScript