Usually under a good light you can look into the rear of the barrel and see leading.
It's usually worse at the rear.

Best way to clean the barrel is to buy a Lewis Lead Remover Kit from Brownell's.
This uses a brass screen to pull leading out, and works far better then using an over-size bore brush.
The kit also includes a special cleaning tip that's uses to clean the forcing cone at the rear of the barrel.

To clean the chambers of lead, buy a bronze chamber brush from Brownell's.
This is not only a larger brush then a bore brush, it has extra stiff bristles.
Usually one or two passes will remove all leading from chambers.