leaky skylight repair

I've got a leaky skylight (or maybe a "roof window"--it's not bubble-shaped) that needs fixing. It's not a common leak around the skylight due to flashing or shingle issues; rather, rainwater leaks between the sash and the glass (and the gasket?) in one corner, where a bit of "fog" also appears. Then of course it drips onto the ceiling and into the room. The skylight was installed as part of an addition about 10 years (and 2 owners) ago, and I don't know the brand (possibly Andersen, which the windows are) or anything about a warranty. So I'd rather just fix it if I can--but how? Do I caulk the exterior? (And if so what kind of caulk?) Disassemble part of the sash or frame and caulk inside? Replace the gasket? Any suggestions would be much appreciated. Thanks.