Short of tearing out the window and replacing it, what can be done to determine the actual source of the leak, and how can it be fixed?

Ice dam is usually caused by improper attic ventilation. This article explains the causes and prevention of ice dam.

Your roof over your home is being damaged by ice dam. Ice dam can cause bad roof leaks and ruin the inside of your home.

Snow & Ice Shield Protects Your Home. The cold temperatures and build-up of snow and ice can cause a wide array of roofing problems.