There is no need for this feat - bards can already do it!
This is actually addressed via a series of tweets by Jeremy Crawford.
Your voice, song, or oration is basically your verbal component. Strumming an instrument, playing a drum, dancing encompasses the somatic component for bards.
Based on this it would be logical to assume this could already happen in everyday interactions with a bard without the need for a feat.
For example, without more actual specifics on how bards cast spells, if you were observing a bard playing a lute and singing fancy to the barmaid, how could you tell the the difference from:
- Bard casting prestidigitation to make the air around him more aromatically romantic
- Capable peasant who knows how to play a lute and knows how to sing.
It would come to roleplay of the bard (up to you DM if you would allow a perception check). In all honesty, unless you were encountering another bard who knows the nuances of your trickery or some crazy fan who's watched you perform this trick before there's no reason to let the basic NPC have a perception check. They have no understanding of the magic to understand if anythings suspicious. The more experience an NPC has with a bard and bard charms the more cautious it might be but in the end there's not way to tell their performance apart from a generic performance based on RAW outside of any physical spell manifestations that may produce itself afterwards.
To that end I refer you to this Sage Advice column from September 2016:
Do you always know when you’re under the effect of a spell?
That being said, an NPC fooled by this already could be suspicious. In that event it would make sense to always give that NPC a perception check because they've been burned by it before. A naive maiden however you might rule doesn't get one at all. It really depends on the world you and the NPC's experiences.
As for is it balanced. That's hit or miss. You could argue since sorcerer has the ability to do so with silent metamagic the bard could. But this is controlled and managed by a resource and eventually that resource will run out. I'm hesitant to allow 'absolute' feats because
- They don't always make sense. (Why can't another bard pick apart my performance).
- They diminish roleplay. If I know this ALWAYS works then it can very easily be exploited.
- Fool me once shame on you, fool me twice shame on me? A feat like this doesn't address this concept. There's no way I would not give a perception check if this was used against an NPC once already. They're already on alert at this moment for this type of behavior.