Shows the breakpoint in the upper left corner.

dev-breakpoints.css

body::before {
    position: fixed;
    top: 0;
    left: 0;
    z-index: 100000;
    box-sizing: border-box;
    display: block;
    padding: 5px;
    font-size: 12px;
    font-family: sans-serif;
    background: #fefaa5;
    border: 1px solid #fff628;
    content: 'xs';
}
@media (min-width: 480px) { body::before {content: 'sm';}} /* $screen-sm-min (bootstrap) */
@media (min-width: 768px) { body::before {content: 'md';}} /* $screen-md-min */
@media (min-width: 992px) { body::before {content: 'lg';}} /* $screen-lg-min */