Click anywhere on your page to show crosshairs that reveal the vertical + horizontal midpoint.

jQuery.showCenter.js

$(function() {
  $('body').one('click', function(event) {
    var halfHeight = window.innerHeight / 2;
    var halfWidth = window.innerWidth / 2;

    var horizontalRule = $('<div style="width: 100%; height: ' + halfHeight + 'px; border: 0; border-bottom: thin solid blue; position: absolute; top:0;left:0"></div>');
    var verticalRule = $('<div style="width: ' + halfWidth + 'px; height: 100%; border: 0; border-right: thin solid blue; position: absolute; top:0;left:0"></div>');
    $('body').append(horizontalRule);
    $('body').append(verticalRule);
  })
})