While Mono Check is active, the filter caused by mono-summing will be displayed as an additional yellow line inside the spectrum display. A more or less static filter shape indicates that the mono-summing will lead to spectral problems (e.g. comb-filters), while a constantly varying filter shows that summing to mono will not lead to any static filtering effects.
You can observe the filter while making changes to your mix to monitor the impact on the mono compatibility of your track.