Mind you, the character probably notices the sorcerer throw the orb his direction just before it discharges in front of him. And if the stone breaks apart upon being destroyed, we can assume a few shards hit him. That would be automatically noticeable.

Even if those somehow escaped his notice, he'd notice within one hour when his regeneration failed to work. (As has been mentioned above.)
I was referring to stealing the stone, much quieter than destroying it. You might still get away with destroying the stone without the wearer noticing (the orb that hit it would look like it was aimed at the character himself, and the breaking itself might go unnoticed in the general chaos of combat), but it'd be a much lower Spot DC.

And he'd only notice the lack of regeneration if he were injured at that time, which he might not be (especially if the stone is lost in a non-combat situation, as is likely for theft).