I was trying to cast prestidigitation on a guard behind his back and my DM said that he would definitely know I did it because the verbal part of the spell would be in a loud voice. He also said that the somatic part would be flashy and with grand movements of the hands. How loud does the verbal need to be and how flashy are the gestures?
[RPG] How loud or flashy are verbal and somatic spells?
dnd-5espell-componentsspells
Best Answer
It is up to the DM, but they are noticeable.
In 5e nowhere does it define anything specifically defining the volume of spells or how grand gestures have to be. These kinds of details are left entirely up to the DM to decide.
This is all the detail we get from the rules. It is worth noting that the rules do even say that there is a potential range of how obvious the somatic component might be, so it may even depend on the spell. In the end, your DM will have to decide how they want it to work in their world.
Jeremy Crawford has shed some light on the intent for verbal components though (in an old unofficial tweet):
One thing is sure though: both the verbal and somatic components are definitely noticeable however. We know this because there is at least one class feature, a Sorcerer's Subtle Magic metamagic, whose only purpose is to make them not noticable: