My first brew has been in the primary fermenter for three and a half days (brewed Saturday night, it's now Wednesday morning). It's in a plastic bucket fermenter with an airlock. There are no bubbles coming out of the airlock, although there's a slight yeasty smell in the cupboard it's sitting in and the lid is bulging a bit. Through the translucent sides of the bucket I can see a two to three inch scum ring, but I can't tell whether that's krausen or just the remains of the foam head after aeration. I haven't opened the lid to check. The temperature in the room has been between 16c-19c.
What should I do?