The margin math assumes every bed masks equally; a live measurement checks whether this one actually does. While playing, the studio compares the bed against the voices across the speech band (200 Hz–8 kHz, weighted toward the consonants) and calls the result even, thin or exposed. There is no separate scope to watch: the verdict is folded into the suggested margin and into the masking status line.
The advice holds the weakest stretch of the session, not the prettiest moment: a bed with quiet gaps is judged by its gaps. If the status still warns at a suggested margin, the bed has a hole where speech lives: pick a fuller bed or raise the music.