Drill dust can add a clue about the material you have reached. Colour, texture and resistance belong alongside the wall's construction and fixing instructions; none is a unique identification on its own.

Do not drill an exploratory hole to discover whether a location is safe. Hidden services and hazardous materials must be considered before the bit touches the surface. Read What Lies Beneath first.

In this lesson, observations come from an already justified fixing task. An unexpected layer, void or metallic sound is a reason to stop and reassess, not press on until the answer becomes obvious.