I can see them unless they don't want to be seen, and the only way to really tell if a person has one or not is if the Shadow is there, and if they want to be seen. What they look like... uh... you know the art of a guy called H.R. Geiger? He's probably one of the people who could see them but didn't know what they were, they look a lot like his art. Mine? Kitten? He's snake-shaped, mostly, but he's got legs like a millipede and feelers like an insect, and I'm pretty sure he's got compound eyes.
I can travel through their dimension, so can my mother, and I can take other people along as well, the new tattoos are my mark as a Herald, so they do represent Kitten, yes.
And don't be sorry for being curious, I much prefer the questions to just being assumed as crazy.