Detecting overflow on floats

How would I be able to detect overflow on these floats?

Where you would see a difference whether it is set to visible or hidden.

overflow: visible;
overflow: hidden;

Don’t all floats contain overflow?

Why am I not able to detect it?

On all my other floats I was able to detect the overflow by removing overflow:hidden;
On this code I’m not able to detect where overflow is.

Is there a way where I would be able to visually see where it is?

.container {
  width: 936px;
  padding: 25px;
  margin: 100px auto;
  border-radius: 25px;
  border: 2px solid #0059dd;
  background: #000000;

.container-top {
  position: relative;
  height: 310px;
  margin: 0 0 45px 0;
  border-radius: 25px;
  border: 3px solid #0059dd;
  box-sizing: border-box;
  background: url("") no-repeat 0 0;
  background-size: cover;   

.container-left-video {
  margin: 0;
  float: left;

.container-right-video {
  float: right;