Spawn Detector Mod 1.11.2, 1.10.2, 1.9.4 is a liteloader mod based on a feature in Not Enough Items. It is designed to show a yellow “X” over blocks where the light level will allow mobs to spawn during the day (ie. above ground in the overworld) and a red “X” over blocks where mobs can spawn at any time. This mod requires LiteLoader to run.
By default, you press “L” to toggle the overlay, though this can be adjusted in the controls settings of minecraft.