[nh1]

Originally Posted by Hunter

And anyway to know that you're in a genjutsu doesn't help you.

So it seems at least against Tsukiyomi, although it helped both Kurenai and

Spoiler:

Shikamaru.

Actually, we do have information about that,

Spoiler:

there are 5 kind of genjutsu which attack the 5 senses.
Tayuya's genjutsu works with sound, if you heard them, you're taken into it, the Tsukiyomi is a dojutsu, if you see the pupils, you're taken into it.
Simple as that.

I don't think it is... I think the Sharingan user has to look into your pupils as well. If someone happens to see his eye, they shouldn't be caught.