Leaky windows: How to know where the leak is?
I would suggest the opposite of wallyk. Instead of trying to back track the leak, which probably goes through sealed off areas, try to reproduce it. Dry it, then use a hose on different parts of the outside window, once you have identified what part of the window is admitting the water, look for any ingresses. You basically need to divide the possible areas the water could be coming entering into zones, and then test each zone separately. Once you KNOW which exterior zone is responsible, then you can look for cracks.