eyeball, my son...eyeballs.
a practiced eye can easily achieve a ±1° vertical on the front sight post.
alternatively, a precision square works perfectly. use the square zip tied to the front sight base with a short , slotted block in between the sight riser and the square.
i have also seen guys take a block of aluminum and thread it for a screw that is the same theard as the sight post. the block is threaded in place where the post goes, and a precision level placed on that aluminum block.
if you just barely start the barrel in the trunnion, you can still make rotational adjustments before running it home.