Most brown besses had dovetail markers engraved on the barrel, and the charleville had a brass front post to be fair. Some muskets even had iron sights like the light infantry pattern brown bess and all rifles had sights.
Also all of the manuals of the time mention aiming down the piece.
light infantry pattern, the image is too large to show the front post, but here's the rear one.
last one is just a prussian rifle.
Slip the left hand along the sling as far as the swell of the firelock; bring the
piece down to the present, stepping back six inches to the rear with the right
foot, and look steadfastly along the barrel of the firelock.
Even with bare muskets you had to look down the length of the barrel, contrary to the myth that soldiers looked away from the flash of the pan.
Yet another musket with sights.
Jäger-Karabiner M. 1807