Imagine the following situation: a College of Lore bard is battling against a sorcerer who has learned the Subtle Spell metamagic option.
The Lore Bard goes first and casts a high-level spell, which the sorcerer attempts to Subtle counterspell with a lower level spell slot.
Since counterspell only has a somatic component and the Subtle Spell metamagic option removes that, the counterspell is not detectable by the Lore bard. The sorcerer also has to roll an ability check (as part of counterspell) in order to successfully counter the bard’s spell.
However, could the Lore bard still use his Cutting Words feature to remove a Bardic Inspiration die on the sorcerer’s counterspell ability check, even though the Bard does not know that a spell is being cast?
The same question also applies to other spells with only verbal/somatic components (which can be Subtle Spelled) that include an attack roll or ability check to function, such as dispel magic or chaos bolt.
Best Answer
Yes, you can use Cutting Words even against a spell with Subtle Spell used on it
Counterspell's trigger is:
If used, the sorcerer's Subtle Spell metamagic option effectively makes it impossible to perceive the casting of a spell with only somatic and verbal components - and thus impossible to counterspell.
Unlike counterspell, however, the College of Lore bard's Cutting Words feature doesn't require seeing a creature "casting a spell. The feature description states:
Cutting Words doesn't rely on you seeing someone "casting a spell". It doesn't even rely on you seeing someone "making an attack roll/ability check/damage roll", because those are simply mechanical abstractions of what's actually happening in-universe. The primary things it requires are that you see the creature and that it be within 60 feet of you.
As a result, you can basically attempt to use Cutting Words on the attack or damage roll or ability check of any creature you see within 60 feet.
Of course, your DM may houserule that you need to be able to visibly perceive that the creature is doing something in order to react to it, but that's just up to your DM (and the players).