I know you can spare them in VATS, but from reading around, they can also appear during the use of VATS without noticing. Is this true? So guaranteed hits without triggering the critical-bar in VATS? On the other side, wearing luck armor would dramatically boost you char then.
Outside of VATS, the pling sound tells you that a critical hit happened, but people claim that it is completely separate from all luck-based perks (critical damage improvement for instance).
Wikia is sadly very vague on this topic. Almost no "in-depth" info.