Is the wheel an invention? What about fire, airplanes, and chemicals? How can you tell whether something's an invention or a discovery? Well, you don't have to.
Discovery seems like passive observation, but to discover beyond the obvious, you have to actively examine. And methods of examination–microscopes, wind tunnels, apparatuses–have to always be invented. On the other hand, an invention could be a dream, but only becomes successful if it can be turned into reality.
Let's examine the relationship of inventions and discoveries further.
Consider two materials that initially appear to be similar. When you invent, you notice that one works better for your requirement, thus revealing differences between the materials. At the same time, discovery of these new properties leads to making more inventions possible.
Inventions and discoveries are two sides of the same coin.