Scale insects are small insects of the order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the superfamily Coccoidea. ( I couldn't say that any better- thank you Wiki). I think it wins the name of super-family since there are over 8000 kinds of scale. - wonderful. Scale insects vary widely in size (really tiny up to baby fingernail size) and colour- might be brown, hard ovals or look like a collection of undersized cotton balls.
Typically, the first thing you might notice about a plant or tree infested with scale will be that it's dripping sticky sap. I have a very large schefflera that - last winter- had a very large infestation of scale and to my embarrasment, I didn't notice anything wrong until one day when I walked past it and sort of stuck to the floor. Dripping sap from any plant usually means bugs are a problem and sure enough, once I put my glasses on I could see many small brown bumps on both the upper and underside of leaves. Not sure if its scale? Just see if it easily scrapes off with a fingernail ( not really as gross as it sounds).
I know that some people will use pesticides to take out scale, but you don't really have to and they are often not every effective, anyway. Just carefully inspect your tree and scrape off any scale that you see. You might need to do it a few times to get all of them and you can wash the sap off with water. If you're squeamish, you can use a Qtip to scrape them off.