Having spent some (a lot, actually) of my audio career as both a systech and a mixer, I used to occasionally wonder if my ears were “on”. My solution was a solid set of IEMs and a portable HD/FLAC audio player with balanced outputs — so I can IEM listen and send the same source to the system. I have a set of 10 songs I am super-intimately familiar with that I can play back as a musical hearing test and system reference source. I know EXACTLY what those tracks sound like (to the point of “the snare on this one should sound woody”, and this one has a vocal that’s “slightly too much sibilance”) so I know if my ears are working properly. FWIW , they usually are, but every now and then…