You can often tell a metal from a non-metal by its physical properties. Most metals are shiny solids at room temperature. They have high melting points and high density, and they are good conductors of electricity because of their delocalised electrons. Most metals are also good conductors of heat.
Most non-metals have low boiling points. Many are gases at room temperature, such as oxygen and nitrogen, and others are dull solids or a liquid. Most non-metals are poor conductors of electricity because they have no free electrons or ions to carry charge. Non-metals that are solids are often brittle, so they break when hit rather than bending.
These are general rules, so there are exceptions. Graphite is a non-metal that conducts electricity, and mercury is a metal that is a liquid at room temperature.