How to tell what the light level is at a given location? Minecraft?

Press f3 you should find it somewhere in that mess

Press F3 (or SHIFT+F3) and look at the values presented. BL stands for "Block Light", which is the light level given off from nearby blocks (like torches, Glowstone, and the like). SL stands for "Sky Level", which measures the exposure of an outside area to the sky; outside is always 15 SL, but this number will decrease as the sky gets obstructed from the area (by making a roof, entering a cave, and other such actions). RL stands for "Raw Light"; this number will display either the value of BL or SL, whichever is higher.

Note that the displayed values are for eye level only. The block where mobs might spawn is one block lower, at ground level. If your torches are on the walls, you'll need to subtract 1 from the displayed BL value to see if mobs can spawn there. If your torches are on the ground, then you would add 1 because the light level would actually be higher at ground level than up at your eyes.