The laser spot test is good for photographing, but without any motion you can’t determine where things are going wrong. If the spot would travel a line slowly for a few inches you could better determine what was going on.
Yes, this is similar to what I was thinking. A spot test will have the galvos always come back to the same exact spot, and that one spot might happen to have dust, or maybe none at all and give a false sense that things are clean. It would be handy to have a spot test where the beam would slowly go to a square that is near the ends of travel, or even multiple size squares to roughly assess the whole scan area. A moving test would allow the beam to hit different spots on the mirrors and could easily prove that a dirty mirror is the issue. It might even be able to nail down which mirror has the dust spot, because on a square, only one galvo would be used at a time. The travel should be slow enough to see it by eye, so something like one second to make a line across the 5" travel would be ideal.