As Brian said, you can scrape them off easily with a fingernail. You do have to look carefully, as you may already know, because they can blend in so well with the bark! (Nasty trick!
)
I also like Carl's soap solution for immature scale. (Have some sitting in the basement right now; it keeps well.) Once scale mature, tho, they develop a carapace that sheds insecticides. So you're looking at either smothering them (a spray oil) or a systemic insecticide. I use a Ferti-Lome product with imidacloprid as the active ingredient. Many experienced growers swear by the Bayer product that also contains imidacloprid (say
that five times, really fast!)