Yep, I use magic marker also.
Also from personal experience I can attest to the fact that you sometimes have to physically pick the hen up and get her off the nest to eat, drink, and poo. I was beginning to suspect that the hen, a first time broody, wasn't leaving the nest to do her business. After two days I pushed her off the nest to discover that she had indeed relieved herself on the eggs and then it looked like she had rolled them around in it until it was caked and dried on like concrete. I did my best to get as much yuck off the eggs as possible but even then I knew it was going to be a problem. Sure enough 3 eggs never hatched and had chicks dead in the shell. A forth chick died at 3 weeks from failure to thrive. I can only point to the soiling of the eggs as the cause behind the chick death.
From now on I will physically make the hen get off the nest the 2nd day after she begins brooding and every day after that. Hens, especially new broodies have no time sense at all. All they want to do is sit on those eggs. Yes it is extra work for the owner but if it gives me viable chicks I'll do it gladly.
I tried to move my broody from her nest this summer she wasn't having it. She use to be so sweet but I tried to move her and she bit me repeatedly. Now I only move my broody hens at night.