Neither is a fully formed grammatical question.
If you're asking about people in general, try something like "Which hand do you wear a watch on?" or "Which hand does a watch go on?"
People typically wear it on their non-dominant hand, so it of course varies from person to person and there's no "correct" answer.
If you're asking one specific person how they wear their watch, then ask them "Which hand do you wear your watch on?"