Some ‘scales’ or marks will be the plants version of a scab, where it’s been damaged and has ‘scabbed’ over, the insect scales do look a bit more ‘stuck on’ and ‘foreign’ as in not created by the plant, if that makes sense? They can also be picked off, and the appearance is different to the plant surface.

I’m currently fighting scale insects on one of my houseplants (I think I’m winning), the adult females form the scale, and the nymphs/larvae are ‘crawlers’ (little white, woolly lines about 1-2 mm long), both can be physically removed by scratching them off, but the scale is tougher and water runs off it

This has some pictures of scale insect pests, and they look very different to plant tissue (and don’t forget, they proliferate very quickly, so tend to show up in large, noticeable groups). Another thing is that the surrounding plant tissue will look yellowy/stressed when under attack instead of it’s usual healthy, normal green