The easy way to tell, just put the bottom of the block against the top of the barrel against the barrel should and see where the center of the back set screw channel aligns with the gas port on the barrel.
As stated, the back set screw channel on the block is also used to produce the gas block channel in the receiver, so where the center of the block thread channel aligns to the gas port, is where the block should align to the barrel.
Also to point out, if the barrel is dimpled, then what should have happened is the dimple is dead center below the gas port on the barrel. So forget about any front dimple towards the muzzle if your barrel is dimpled front and back, and make sure that the set screw on the back of the block is mated to the dimple closest to the barrel shoulder its installed.