How can I determine where mobs will spawn?
This is a feature of Not Enough Items (NEI) as of version 1.4.7.0. A red grid showing blocks mobs can spawn on can be toggled on and off with F7 (by default; I like to rebind it to L for easier access).
The best mod that does this is called Monster Spawn Highlighter made by Lunatrius.
You can find it here
Usage is simple. Hit L, click on mobs to select those you want highlighted and select the type of highlight (bottom left). Finally click Done.