Press F7 and the light level shows up on the ground around you, colored by whether monsters can spawn there. It makes lighting up a base or clearing out a cave a lot easier.

These are in the assortedlightoverlay-client config file. They only change your own game, so everyone can set their own.
lightOverlay.radius is how many blocks around you the overlay reaches. 16 by default, anywhere from 4 to 32.lightOverlay.showSafe shows the green spots where monsters never spawn. On by default. Turn it off to only see the unsafe spots.Light Overlay also comes in Assorted Util with the other five Util mods, if you'd rather have them all in one download.
The full guide is at assortedmods.com. It's also in the game, in the Instruction Manual from Assorted Lib.
Found a bug? Report it on GitHub.