Often you can identify a material as a metal or a nonmetal just by looking at it. For example, you can tell this foil is a metal. Metallic luster is one characteristic of metals, but if you're asked to distinguish between metals and nonmetals on a test, you're going to need more than one difference between them. Here's a nice extensive list of differences between metals and nonmetals to help you tell them apart based on physical and chemical properties.
Are there any properties I missed? If so, please post a reply.