How easy is it to recognize that a creature is under the Dominate Monster spell?

Question

Had an interesting session with a DM I generally quite like playing with where he made 2 decisions I disagreed with. Unfortunately a bit of research hasn't clarified if I'm misunderstanding the rules, so I thought I'd ask.

Scenario: we encountered a frost giant and a few stone giants who weren't yet hostile to us, but not friendly and refused to cooperate. I cast dominate monster on the frost giant (the leader), making him listen to us in the first instant, while another party member tried to convince them to help us. I then took full control of him, and got him to tell the other giants that we were friends (all of this happening telepathically, as per the spell). At this point the DM said one of the stone giant (a dreamwalker) recognised him as being charmed (apparently because he looked dazed and said that we were friends randomly), and immediately attacked the frost giant, forcing another save and releasing him.

The justification here was the stone giant dreamwalker is familiar with charms (it has a passive charm power that does not allow a save on damage, so works quite differently to dominate monster). I left it when he made the call but then after the game finished argued that because the spells are quite different I doubt she'd have recognised it, and even if she did, she'd be more inclined to attack me than her leader the frost giant.

How easy is it for another NPC to recognise that a creature has been put under the dominate monster spell, after the spell has been cast and the verbal/somatic components not noticed?

And would they think that directly attacking that creature would be the most reasonable course of action, barring metagame knowledge of saving throws?

A dreamwalker has an intelligence score of 10 and a wisdom of 8.

Answer 1

"Who are you, and what have you done with our leader?"

Thomas Markov talks about the RAW regarding a spell's casting/effects/etc being perceivable, but I'm going to argue that the issue wasn't the visibility of the spell itself.

Consider these three quotes (emphasis mine) from your question:

... we encountered a frost giant and a few stone giants who weren't yet hostile to us, but not friendly and refused to cooperate.

I then took full control of him, and got him to tell the other giants that we were friends

... the DM said one of the stone giant (a dreamwalker) recognised him as being charmed (apparently because he looked dazed and said that we were friends randomly), and immediately attacked the frost giant, forcing another save and releasing him... The justification here was the stone giant dreamwalker is familiar with charms (it has a passive charm power that does not allow a save on damage, so works quite differently to dominate monster).

Now I'm not familiar with exactly what was said during these exchanges, or how much stock your table puts in the balance between player and character skill, but two things jumped out at me immediately.

1. The giant was unfriendly and uncooperative, then suddenly turned cooperative and even tried to introduce the party as friends? That's suspicious enough on its own.

2. Unless the ability is completely unconscious (which it could be, some passives are), the dreamwalker has a charm ability of its own. Even putting aside the assumption that creatures with abilities know how they work, in practice if not in theory, the dreamwalker at the bare minimum knows charms exist and roughly what they do - not necessarily how to break said charm, but enough to know that they need to try something: attack, raise the alarm, etc.

You listed its ability scores as being low, but 10 and even 8 aren't that low. Even comparing them to the 15 you get from standard array, an 8 only reduces your chance of success by 15%. Bad, but not crippling, and doesn't preclude specialized knowledge in any event.

We have seen several examples across multiple media of unusual character behavior being a sign that a creature has been anything from reprogrammed to killed-and-replaced, enough to give us the phrase and the trope that opened this answer.

Answer 2

It's up to the DM.

The Player's Handbook has some guidance, but it is guidance, not rules:

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless a spell says otherwise.

This particular guidance is going to depend entirely on a DM ruling. Xanathar's Guide to Everything contains some additional guidance, mentioning a spell very similar to dominate monster:

Other spells, such as charm person, display no visible, audible, or otherwise perceptible sign of their effects, and could easily go unnoticed by someone unaffected by them. As noted in the Player’s Handbook, you normally don’t know that a spell has been cast unless the spell produces a noticeable effect.

Xanathar's Guide here provides further guidance, and establishes that it is entirely reasonable for a spell like charm person to go unnoticed. But again, guidance, not rules.

The reasoning for your DM's ruling sounds pretty reasonable to me. I can definitely understand that it might a feel a bit cheap for your 8th level dominate monster to get stopped like this, but I also believe it isn't an unreasonable call for your DM to have made.