Missing eggs is the most obvious sign. With some, they just take the eggs from under the hen and leave the hens alone if they can get enough to satisfy them. Sometimes if they come to visit and there are no eggs they will pull the egg ducts out of the sleeping hens. I have heard of people going in to collect eggs and the hen looked like it was just sleeping in the nest in a normal position, but when they tried to move it to collect eggs, it was clear it was dead. They will also sometimes just kill the hens in a more messy manner, usually ripping them open from the vent are. These are just things I have heard. I only lost one hen to what I THINK was a skunk. Her death was the messier type with her belly torn open and her head missing. I would examine the coop/run very carefully and look for signs of entry. Hopefully it's just a lull in the laying cycle, but I would investigate if I were you.