How do geologists determine wether it is intrusive or extrusive?

Geologists determine whether a rock is an intrusive rock or an extrusive rock by examing their crystal/grain properties. Intrusive rocks, due to slow cooling of magma below the earth's surface, has larger crystal/grain texture and can be seen by the naked eye while the extrusive rocks, due to rapid cooling of magma above earth's surface, has fine crystal/grains, which cannot be seen by the naked eye.