Some would might dare saying weakspots are waste of time. But the simple laws of the physics says that "an object is only as strong as it's weakest point".
So here's some generel rules about weakspots.
When I learned playing the game, I was told one simple rule about weakspots: Everything, that sticks out. That can be anything. Machine-guns on the turrets, lights, hatches, viewports and gunports.
There are also other areas of the tanks, you can try to go for.