The Witch Sight warlock invocation reads:
You can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of you and within line of sight.
If you are looking at, say, a creature under the effect of true polymorph, are you aware of how the creature appears to others? In other words, if a humanoid polymorphs into a bear, you perceive the humanoid but do you know it is currently in bear form, or do you need to ask a nearby bystander?