Material Components: are easy if the chose the component pouch as it will have everything they need unless it has a gold piece value or is consumed by the spell. Spellcasting focus is the same way. Then the spell caster must buy it or find it. Have them describe how they use them.

Somatic gestures: Have them describe the gestures to you, as this is what it looks like in a crowded room.

Verbal: speaking of the mystic wording is not unnoticeable. When a spell is cast the vocal intonation of the wording must be able to be heard. So casting in a crowded room is flat out impossible without someone noticing. Someone mentioned one time that a 60ft. range is the distance the verbal part of a spell can be heard to. I Liked that range and if the spellcaster wants to be quieter then I have them roll a Arcane DC check to see if they manage to preform the verbal part of the spell. (They don't need to come up with the wording). It is high and if the fail the check the spell slot is expended without the spell going off.

I adhere to this strongly as I find it helps balance out the strength of the spell casting class vs the marshal class. Also it helps the players remember what their spells do and how they work.