The way I do it is to listen at the volume I normally listen at.
P.S. That may be different levels on different loudspeakers.
Evaluating your speakers
Many, many years ago I recall reading an article that the best way of evaluating your speakers' quality was to play them at a very low volume and hear if the bass, midrange and treble are in the same proportion as when played at medium volume.
Medium volume being defined as not loud enough to induce room acoustics.
My OHM Walsh 4's meet this criteria when placed on a hard surface (vent is on the bottom of the cabinet), not so much when placed on thick carpet.
Anyone recall reading something similar?