Magma location
Is there a way to know where the magma pool is (that preferably doesn't involve mining in random directions)? If not, does magma usually exist on a certain depth or something?
You have two sources of guaranteed lava in Dwarf Fortress -- The first is volcanoes. Embarking on a volcano will guarantee a ready source of lava.
The second happens during world gen when you allow / deny the existence of the magma sea (default is on). On worlds with a magma sea, you will hit magma as long as you keep digging down, past the third cavern level.