FLAC keeps replay gain information in the following low level FLAC comments:
REPLAYGAIN_ALBUM_GAIN, REPLAYGAIN_ALBUM_PEAK, REPLAYGAIN_TRACK_GAIN, REPLAYGAIN_TRACK_PEAK
In order to normalize the names across all supported audio file types, Yate calls the fields:
ReplayGain Album Gain, ReplayGain Album Peak, ReplayGain Track Gain, ReplayGain Track Peak
If you load a file into Yate and position to the UD Text editing panel. (Info, Artwork, Audio, etc.), you will either see or not see these items. You can also do an Edit>Snapshot to check for the fields.
In Yate you can write an action to walk your collection and produce a file which lists all tracks containing or not containing any or all of the above fields. Alternately it can highlight loaded files which contain or do not contain the data.
Remember that different applications may encode the replay gain values at different levels. This is configurable in Yate.