How to detect minor leaks in problem area below skylight?
Rather than trying to find the location of the leak, I’d re-seal and re-flash the entire skylight.
I’m sure it’s not that large and if you fix one side of it today, then another side will have a problem next year.
I’d follow the manufacturers installation instructions if you have them. If not, I’d use “peel and stick” tape on all sides and make sure the tape correctly laps previous laps.