What indicates that a drain line is blocked?

A drain line being blocked is typically indicated by water backing up in sinks, tubs, or toilets. This occurs because, when a drain line is obstructed, wastewater cannot flow freely through the system. Instead of being evacuated through the drain, the water accumulates in the fixture itself, causing it to overflow or back up. This blockage disrupts the normal drainage, making it the most direct and observable symptom of an issue within the plumbing system.

Other symptoms, such as strange noises from pipes or low water pressure, may indicate different plumbing issues that do not exclusively relate to blockages. Similarly, foul odors from faucets could be due to numerous factors, including decaying organic matter or stagnant water, rather than a direct blockage in the drain. Therefore, the presence of water backing up is the clearest and most definitive sign of a blocked drain line.