Flutter echoes can be detected by a handclap test while others listen.

Evidence

The classic demonstration is the “handclap” test. Stand between two reflective parallel walls, clap your hands and (especially to the person clapping) there may be a disturbing flutter echo: the clap rattling back and forth between the walls. This is an interesting demonstration, but the real problem is when the “clap” emerges from a loudspeaker and is heard as a flutter echo to a person sitting in the audience. So, the first test involves someone moving from speaker to speaker in a system, clapping the hands, while someone else moves around the listening area to see if there is a problem. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1030