How do you know if a sample of matter is a mixture?

A substance is scientifically defined as matter the has the same composition and properties throughout, so substances are elements and compounds. Mixtures are not considered substances. However, if we are taking the Word substance to simply mean "something" as in everyday (and not purely science-based) use, then one could use the following criteria:

A compound is a substance whose smallest unit is made up of atoms of more than one element chemically bonded together, such as carbon dioxide (CO2), water (H2O), sugar (C6H12O6), and ammonia (NH3). A pure sample of a compound will appear the same throughout, and always occurs in specific proportions of its constituents. For example, pure sugar is always composed of molecules of C6H12O6 and always occurs in that proportion--6 atoms of Carbon, 12 atoms of Hydrogen, and 6 atoms of Oxygen. Compounds can only be formed or broken down through a chemical reaction.

A mixture is a combination of substances (elements or compounds) can either be homogeneous or heterogeneous. Heterogeneous mixtures do not look the same throughout, so they are easily distinguished from compounds. Homogeneous mixtures (or solutions), like tea, Kool-Aid, blood, brass, and sugar water, are only physically bonded together and can be separated through physical means.