How can I tell if I'm poisoned or have a disease?

Go to magic under active one, at the bottom. Red = disease

You have been introduced to one of the UI shortcomings (one should be reminded in a much more straightfoward manner)
But NPCs will also comment on how you look sick - as in ill, not stylish.

*edited for clarity*

Magic
Active effects
Rock joint etc

go your local clinic and for god sake wrap up when you are prowling for trolls!!