If the gun is a "flat top" and if the gas block has a picatinny rail one the top, you can remove the handguards and turn the upper "belly up" on a smooth flat surface. Adjust the gas block until the upper does not wobble on the surface. The gas block and the receiver will be aligned to the table top.
If the gun has a low profile gas block that does not have a rail on it, you can just eyeball it so that it is in the 12 o'clock position. Just look down the barrel towards the reciever and line it up. Be sure the gun is unloaded of course! It will be close enough to fire, even if you miss the alignment a little.
If the front gas block is a tower type sight, you can still eyeball it by aligning the sight with a sight installed on the receiver, looking to be sure that they are aligned vertically. Just like aligning posts in a barn or when building a fence. If you see a post peeping from behind the one in front of you, one of them is not in line with the other. I don't know if I explained this very well, but that the way I do it. You can temporarily install an old scope ring and align the sight to the center of the scope ring by looking down the barrel towards the reciever.
If your gas block has the set screws in the bottom, you can remove one of them and align the little dimple that is left from where the screw bites into the barrel. Just look down through the hole for the dimple.
Any of these tricks will work, and will get the gun working. I have found that you can miss the alignment quite a bit and the gun will still function fine. If you're using open sights is the only time when it's most critical to align the gas block perfectly.
Hope this helps.
