1. Color Change: A change in color is a very common indicator of a chemical reaction. For example, when iron rusts, it changes from a silvery gray to a reddish-brown.

2. Formation of a Precipitate: A precipitate is a solid that forms from a solution during a chemical reaction. This solid will be visible and may settle to the bottom of the container. For instance, when you mix solutions of lead nitrate and potassium iodide, a bright yellow precipitate of lead iodide forms.

3. Production of Gas: The release of a gas, often accompanied by bubbles, can indicate a chemical reaction. For example, baking soda reacting with vinegar produces carbon dioxide gas, which you can see as bubbles.

While these are common signs, it's important to note that not all chemical reactions have visible signs. Some reactions occur without any noticeable change.