Light a candle for a few seconds. Extinguish the candle by blowing out the flame.
Slowly move the candle around the window frame. Watch the smoke that flows from the burnt wick. When incoming air hits the smoke, it will cause the smoke to fluctuate its pattern. If this doesn’t help, proceed to the next step.
Stand outside the window with a flashlight during nighttime. Turn on the flashlight and point it at the seams around the window. Have a helper standing inside facing out at the same window.
Slowly move the flashlight around the window frame. Have your helper report if any light shines through the frame or wall. This will indicate leaks.
Examine the edges of the window for visible damage. Signs of water damage include rotting wood, holes, peeling or bubbled paint, dark discoloration and streaks along the frame moving downward. Water seeks its own level and will move downward to the base of the frame.
Scrape off damaged paint with a wire brush. This exposes damaged areas and may provide a direction of travel for the water.
Look at the path of travel and the shading of discoloration. Follow the water path upward, paying attention to the changing shade of the discoloration. Where the water first entered or impacted the frame, will be the darkest discoloration. This may be where the leak is.
Seek out rotting, split or wet wood. These are all signs of excess water continuously traveling over the same path. Over time the water seeps into the wood, causing damage.