Detecting Entry Source Of Ceiling Leak?

I have a wall across the washroom which has leaked since it was done. It only leaks when it rains. The leak occurs between a window on the north wall and the outside back door. I need to try to find where the water is coming in from. There is some flashing at the top of the wall outside that I suspect might be the source.

The water appears on the sheet rock low on the wall near the floor and heavy rain makes it go higher up the wall. The water stains are awful looking now. At times I thought it may be coming in from the window frame at the bottom. There is vinyl siding on the outside wall. The window frame is deteriorated and needs to be replaced.

The bottom line is: Is there a way to detect the water entry source? My husband was ill for some time before he died and did not feel like trying to repair it. I have no real knowledge of carpentry and a ear of high places.

At age 74 I am willing to see what it takes, and maybe find someone who would be willing to do the "high place" work. I have done many things on my own, just need guidance and a direction. Many of you have given me very good help in the past. I appreciate any advice any one may have. Thank you.

Oneta