How can a mechanic decide whether to dimple or drill a flush rivet?

The choice of whether to dimple or drill a flush rivet fundamentally depends on the thickness of the top sheet being joined. Dimpling is typically used for sheets that are of a certain thickness that allows for a proper dimple formation, creating a recessed area for the rivet head to sit flush. This process is common for materials that are within a specific thickness range, where dimpling will provide adequate strength and a smooth finish.

In contrast, drilling may be preferred for thicker materials where dimpling may not be effective or might compromise the integrity of the sheet metal. The drilled hole allows for the rivet to fit without needing to form a dimple, which could be infeasible due to the material's thickness or hardness.

Other options like examining the color of the metal, the type of rivet being used, or the age of the materials do not directly influence the decision between dimpling and drilling in a way that thickness of the top sheet does. Therefore, when determining the method to use for flush rivets, the thickness of the metal is the key factor.