stoli,
> I was wondering how to use the clap test to determine your room treatment needs. <
If you hear a "boing" sound or repetitive echoes, that's a good indication you need acoustic treatment. Another, more basic, indication is that you have a room.
> if there's an echo, does one need absorbers or defusiors. <
Most rooms need absorption much more than diffusion. The main feature of diffusion is it avoids echoes without making the room more dead sounding as absorption does. But most rooms are already too live sounding, so absorption is usually a better choice. And all rooms need absorption at bass frequencies.
--Ethan