Is obsidian an instrusive or extrusive igneous rock how do you know?

Obsidian is an extrusive rock and is formed from rapidly cooling magma.

Obsidian is also known as volcanic glass and one can find only sub-microscopic crystals in it. This is because it was cooled too fastly for large crystals to form. If the material that obsidian consists of were an intrusive rock and had a lot of time to cool down, one would find that it would be composed of easily visible crystals.