Easy way to see the Bootstrap grid?

Updated Sherbrow’s answer for Bootstrap 3:

div[class="row"] {
    outline: 1px dotted rgba(0, 0, 0, 0.25);
}

div[class^="col-"] {
    background-color: rgba(255, 0, 0, 0.2);
    outline: 1px dotted rgba(0, 0, 0, 0.5);
}