What tool is often used to check for fire extension in ceilings?

The drywall hook is commonly used to check for fire extension in ceilings due to its specific design and function. This tool is essentially a long, sturdy hook with a sharp end that allows firefighters to effectively pull down sections of drywall, which is often used in ceiling construction. By removing or probing through the drywall, firefighters can assess whether fire has spread into concealed spaces above the ceiling. This is critical in identifying hidden fires that could reignite or spread if not addressed promptly.

The other tools mentioned, while useful in various firefighting tasks, do not serve the same purpose for checking fire extension in ceilings. A fire axe is primarily used for making cuts in doors or walls. A pry bar is useful for prying apart materials but lacks the specific design to pull down ceilings effectively. A sledgehammer may break through materials but is not precise enough for inspection purposes without causing excessive damage. Thus, the drywall hook stands out as the most appropriate choice for this specific task in fire investigations.