How can you tell whether a unknown substance is a mineral

A mineral is any chemical compound with a constant chemical composition, which of course is the definition of a "pure substance".

In earth science you might be distinguishing between "rock" and "mineral". A mineral is a pure substance. A rock is a mixture of two or more minerals in various proportions.

For instance, the rock granite is made up of three minerals: quartz, feldspar and biotite (mica).