Skylights, in essence, are just one of many roof penetrations. When I am on a home inspection and I am looking for roof leaks, I generally find them at the roof penetrations, including the skylights.
So how can you tell if your skylight is leaking, and what can you do to help prevent this issue?
First, be conscious of them. This doesn't mean you have to be staring up at your skylights looking for leaks every day, but you should look up every so often. When you do, keep in mind that moisture stains don't always mean there is a leak. Often times it is just condensation building up, especially if the skylight happens to be in a kitchen or a bathroom.
Also, make your skylights part of your Spring and Fall maintenance. Often times when I am on the roof I see debris built up, organic growth, and other issues that can encourage deterioration and leaking.
In short, skylights tend to require more maintenance than other parts of your roof. But when you give them the needed attention, they will bring you years of natural light and comfort to your home