Look up the rules for Components in the PH. They are clear. There is no "subtle" spellcasting.
Here are the opening/relevant sentences for the description of each type of component:
- Verbal - Most spells require the "chanting" of mystic words. (Chanting isn't subtle.)
- Somatic - Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. (Forceful gesticulation isn't subtle.)
- Material - Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5, “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell. A spellcaster must have a hand free to access a spell’s material components — or to hold a spellcasting focus — but it can be the same hand that he or she uses to perform somatic components. (Having to manipulate the component pouch or implement isn't subtle.)
In my games, there is no such thing as a "subtle" spell unless a special ability allows for it, like Metamagic. No slight of hand, no stealth, nothing.
- All Verbal components are clearly spoken and audible.
- All Somatic components are clear hand gestures.
- All Material Components/Implements are at hand and manipulated.
- Sense-obscuring factors, or the range from which the spell is cast if it is far away enough, may be relevant, as they could affect sight and hearing of the spellcasting.
Strict component requirements is one of the most important balancing factors of spellcasting in my games. Enemies have to follow this too. Don't want to be accused of charming someone? Don't cast a charm spell on them.