What is the best way to tell a chemical change has taken place?

Question

1. The matter changes color
2. The mixture separates into layers
3. The composition chnages

Answer 1

I think the answer you want is 3 but color changes and separation into layers (if the original mixture was homogeneous) often indicate a chemical change has taken place. If the composition changes you KNOW a chemical change has occurred.

Answer 2

I know definitely that it is 3