You would press "P" on your keyboard, the scroll down to "Active Effects". Active effect will show all active enhancements or effects currently in play and anything in red is a negative effect.
To get rid of negative effects you'll either have to visit a shrine of some type or take a potion – Cure Disease.
Also, if you’re sick and around other NPC’s they’ll comment on your health. Thus this is also a good indication that you are suffering some type of disease or illness.