The Solanaceae Family has been highly cultivated for food over the years. This group includes potatoes, tomatoes, eggplant, sweet peppers, chili peppers (but not black pepper), paprika, tobacco and petunias. Some plants of this family have medicinal value, while some, like deadly nightshade (belladonna) are quite poisonous.
The good news is that the risk of becoming seriously ill from eating potatoes, tomatoes, eggplant or peppers is relatively small. Yet for susceptible individuals, the symptoms produced make it worthwhile to avoid these foods. Some people feel better when they give up all foods in the nightshade family completely. Others are more tolerant, only becoming effected when larger quantities are eaten or specific members of the family are consumed.
The nightshade family can be tricky. After eating a tomato, it can take as long as 48 hours before a reaction occurs. And it can actually take as long as six months for all reactions to stop. This means if you want to test for nightshades you may need to completely give up every member of this family for six months. Some respond in a few days, but to know for sure, avoidance may need to continue for a long time. You must carefully check all labels. Prepared foods often have potato starch or tomato paste. Relish may contain peppers or paprika. Anytime you see "spices" listed under ingredients, the food may contain paprika or pepper.