In the beginning I assumed that it wasn't, however later when Suzaku forcibly takes Lelouch to Charles, Lelouch for some reason notices, that the Emperor also has Geass, which makes me wonder if there really is a sigil that is visible.
IIRC, Geass is invisible and undetectable save for to other Geass users and the beings that give the power of Geass to humans. I can't recall what they are called.
This is further proven in R2 when has to deal with the guy who has the Geass canceler. Again Lelouch and his brother Rolo were able to detect him.
I kind of watched a few episodes recently but I did watch both series some time ago.
No the geass symbol in the eye is not visible, otherwise it would be really easy to identify such people. people would freak out if they same someone with such a weird eye and that never happens.
The one weird exception to this is Lelouch who reacted like he could see Mao's geass symbol. Maybe other geass users can see it (unlikely) or maybe Lelouch was just clever enough to understand it (most likely).
The red rings around the eyes of geassed people is similar, it's invisible for the people in the show. That's because it's "non-diegetic" information, which means it's information which only exists for the audience and isn't actually part of the universe. Code Geass usues a lot of non-diegetic information, for example the red sphere in which "time is stopped" by Rolo's geass, or the sphere in which Jeremiah uses his geass, people can't see those either. Non-diegetic information is very common and all storytelling sues that. The easiest example is background music which tells the audience when a scene is sad, romantic, scary ..., there's obviously no music playing for the characters in the fiction itself.