Jumpering a wire across the breaker contacts will prove if that's the problem at least. Just do that and turn it on. Be prepared to turn it off quickly too. It doesn't sound like a burned armature and you say it shows no signs. Usually there is a drop in power and rpm just before it quits if that is the problem. About the only other thing I can think of is a broken wire.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.