If a sound is created in a room and then the sound is abruptly stopped, the reflections will linger in the room for a short period thereafter.
These videos of hand claps, recorded in locations with various types of reflective and absorbent surfaces, show how you can roughly guess the reverberation time by simply clapping your hands. Listen to these a couple of times until you get used to hearing the time it takes for the sound to fade away in each video. Turn up your speakers (but not so loud as to damage your hearing) if you find it difficult to determine the end of the sound fading out.
It’s a good idea for you to get a feeling for reverberation, plus your clients (and your boss) may be astounded (well, perhaps at least mildly impressed) by your ability to estimate reverberation time by simply clapping your hands.
In our anechoic chamber.
Estimated RT60 close to nothing.
In a meeting room.
Estimated RT60 around 1 second.
In a corridor.
Estimated RT60 around 2 seconds.
In our parking basement.
Estimated RT60 around 3 seconds.