Plants can look very similar and be hard to tell apart just by looking, especially when they are small, young plants. I believe S. actinophylla has harder, waxy leaves and a woodier trunk than its safe cousin, S. arbicola. When you buy plants, always go by the scientific name because most plants have many common names, and several different species are called the same common name (like the umbrella tree). S. arbicola is usually called a dwarf umbrella tree, but you can't trust that either.
If you're at a big box store, and they don't have the scientific name on the plant, if you're not sure, I wouldn't risk it. Spend $10 more at a nursery/greenhouse where there are knowledgeable people to help you, and don't risk your chameleons life.