This stylesheet adds text describing the current Bootstrap Breakpoint in the top right corner of the screen.

bootstrap-breakpoint.css

body {
  margin-top: 40px; /* This margin just makes the text easier to read. You can remove it if you want since it can mess with your other styles. */
}

body::before {
  content: "XS";
  color: red;
  font-size: 2rem;
  font-weight: bold;
  position: fixed;
  top: 0;
  right: 0;
}

/* This box class is purely used for explaining how the bootstrap grid system works. */
.box {
  background-color: lightblue;
  border: 1px solid blue;
  min-height: 50px;
  font-size: 2rem;
}

@media (min-width: 576px) {
  body::before {
    content: "SM";
  }
}

@media (min-width: 768px) {
  body::before {
    content: "MD";
  }
}

@media (min-width: 992px) {
  body::before {
    content: "LG";
  }
}

@media (min-width: 1200px) {
  body::before {
    content: "XL";
  }
}

@media (min-width: 1400px) {
  body::before {
    content: "XXL";
  }
}