Following on from this question: Is a creature aware that a Hex or Hunter's Mark spell has been cast on it?
If someone suffers a significant amount of psychic damage, how would it manifest on their character? A barbarian at full health is hit by a succession of psychic attacks, bring his health down by a sixth each time, so by the end he's almost dead. Would cuts or bruises manifest in his skin? Would he recoil when hit? Would an observer notice each hit, or the progression of injuries?
If a bard's Vicious Mockery spell is used on someone in public or during some kind of contest, I'm assuming the victim would be able to discern the cause, but would an observer be able to tell what was happening (and not just think it was a bard shouting insults)?
Best Answer
I have said it before and I am sure I will say it again: In D&D 5e specific beats general.
The Rules
General rule on spell effects (PHB p 204):
So, in general, if the spell has a perceptible effect that effect is perceived; if it isn't then it isn't. However, as always, specific beats general: if the spell says something different then that takes precedence.
General rules on hp & damage (PHB p 196)
hp do not exclusively represent the creatures physical body - the also represent metal ability, bloody-mindedness and luck.
General rules on psychic damage (PHB p 196)
From these, there are no general rules on how psychic damage works.
Advice on describing damage (PHB p 197)
Now, this is advice so how you want to describe it is up to you but according to the guideline: above half hp - no visible sings, below half hit points - visible signs, 0 hp - direct damage.
Specific rule for Vicious Mockery (PHB p 285)
The specific rule is the enchantments are subtle, no one would notice these, possible including the victim (particularly if they save). For describing damage, in the absence of the specific rule you fall back on the general.
The Ruling
Whatever you want is fine so long as you consistently give the players the information they need.
Some people play in meta-game way with hp and treat them purely as numbers with no description applied - "You do 3hp of psychic damage; he now has 8hp left".
Others treat it as pure description with little to no feedback on mechanical effects - "You lash out with your insults, he pauses momentarily and blinks before raising his battle axe and snarling at you."
Or you can take the middle road - ""You lash out with your insults, doing 3 points of damage; he shows no obvious signs of distress."
Whatever works for your group is good.
TL;DR
If you want to follow the advice in the PHB then I would suggest the psychic damage will attack "mental durability, the will to live, and luck" manifesting as glazed eyes, trembling, tears, sweating, unprompted flinching etc. for less than half hp and bleeding from ears, eyes and nose for 0hp.