What the best way to align the gas block?

Many years ago I saw a breakaway plastic rod, which looked okay.

In case it matters, for an Armalite AR10...

Bore Scopes can be found on Amazon for about $50.   Probably the best way.

Bore scope, as suggested already.

Gas Block Genie or a piece of spaghetti noodle are some other favorite methods.

Are you aligning a fixed FSB, aka sight tower, or just a gas block?  If just a gas block I simply put a piece of masking tape on the journal and mark where the port is with a pencil.

If a sight tower I have a different method that is simple and effective that I can 'splain.

Insert a piece of DRY spaghetti noodle all the way into the gas port on the barrel. Break it off flush with the barrel. Put gas block onto barrel and get it all the way back against the stop. Turn barrel upside down and wiggle the gas block until you feel the spaghetti catch the gas block. You must do this slowly, or you will break the spaghetti noodle before everything is lined up. Once the block is lined up and tightened, use a cleaning rod to break the noodle and remove it.

All gas blocks will have a hole in the bottom that is aligned with the gas port (it's how it's drilled).  Measure from the back of the block to the center of that hole (Dim A).

Measure from the front of the gas block buttress to the center of gas port on the barrel (Dim B).

Subtract Dim B from Dim A, this is Dim C.

Mount the block so the back of the block is Dim C from the front of the block buttress on the barrel.

If Dim C is negative, one of the two parts is made wrong.

(If there isn't a hole in the bottom, there is a slot, and you can measure the port location directly.)