How can tell if a material has undergone a chemical change? a physical change?

A physical change is if something to change shape or color. a chemical change would be is something smelled different or something like that.

Physical changes involve states of matter and energy. No new substance is created during a physical change, although the matter takes a different form. The size, shape, and color of matter may change. Also, physical changes occur when substances are mixed but don't chemically react.
A new compound (product) results from a chemical change as the atoms rearrange themselves to form new chemical bonds. Gas released, combustion, or new substance is created