Song says "It Never Rains in California" at least not till everything's dry then burnt when rain falls and mud slides...
Anyhow, There are little Weep Holes in the lower frames of sliding windows, and I suspect your Winne has one on each side of the cabover. If Tracks and/or Holes get blocked with dirt, mold, etc. water will back up and ooze inside. We have those windows, and they were such a pain, we sealed them "closed" with clear sealant.
Pressure Test is your best bet. It's the Gold Standard in RV Leak Detection. Otherwise, we try to inspect but basically we guess. A professional may guess a little better than an RVer, so I suppose they are giving us an "Educated Guess."
Edit: The Front Window is a top suspect too, BUT Clearance Lights contribute much more than their share of leaks! Each is mounted with two fasteners (hopefully screws not rivets) so 10 holes there. Then the wiring takes another hole, now we're at 15 suspects. Shop I use, seals bu pulling the lenses and sealing the Bulbs to the Inside of the Light Base, Where the Lens snaps onto the Base, then Around the Base itself. If we drive INTO a rainstorm at 65 and the rain's coming at us at 35, that's a 100 MPH, Hurricane Force, Wind slamming those drops against the coach.