aramis erak
Legend
It really depends on the DM. Some DMs are going to make all casting obvious to all (minus Subtle spell). Some DMs are going to have all casting be discreet. Most, I think, will go somewhere in the middle.
Personally, I see nothing that requires you to speak loudly when casting a spell, allowing it to be whispered. Somatic components are not necessarily large gestures, and very few would understand what those gestures mean anyway (unless every 3rd person you meet in the world is a caster). A Slight of Hand (Somatic) vs. Perception and/or Deception (Verbal) vs. Insight seems appropriate to me.
A lot depends upon the nature of the spell how I interpret the verbal component.
If' it's got a V - I require it be in at least a firm speaking voice. If the spell has "word of" in the name and is targeted, I require it be loud enough that the sound reaches the target audibly.
It it has an S, the somatic component needs be something readily done with a hand and/or wand.
To conceal the component within normal conversation without abrogating the requirement, I generally require an arcana roll, but allow it to be attempted.