A watch that is stopped: Where to begin
Always assume the battery.
It is the easiest thing to repair and even when the customer “Knows it is new / full and it is something else”, check it. It isn’t wasted time.
Even when I know the battery is full because I have put it in there
I still check the battery.
A few times I have come across a new battery that is dead or other times I have stupidly refitted the old dead battery (usually happens in a busy patch and you loose time for nothing when you can least afford it).