How to show light levels minecraft

Handling this alternative of Checking Light Levels In Minecraft requires the holding of a Redstone torch. With the use of a Redstone torch, light levels can be detected. All the player has to do is to switch the Redstone torch. It lets out the light level of …

There are other answers below:

I show you how to see light levels to know if monsters will spawn or not in Minecraft

The minihud mod can be used to display a light level overlay, this is super useful when lighting up caves and other large areas to make them spawn proof.

Key shortcut to see light levels. I think it would be a great addition to minecraft to optionally see where mobs can spawn (Just like in the Not Enough Items mod) Running arround with F3, is really annoying and I would not consider it cheating when you have a Key shortcut (like F3+L as an example) to save some really unnecessary time.