Are bubbles an indicator of a chemical reaction?
No. Bubbling could indicate that a gas is forming in a solution during a reaction, but it would also occur as you bubble a gas (e.g.
You can tell if a gas is forming since gas molecules tend to take up more space than their liquid counterparts, and since they often are highly energetic. When they move around, they disrupt the liquid around them, creating spaces in the liquid. These spaces are gas bubbles, surrounded by a cluster of liquid molecules.
As that happens, assuming that the gas is less dense than the liquid, the gas molecules float up and out of the liquid, popping on the surface.