What symptoms make you think of foodborne illness?

GI signs. Usually it is an illness with diarrhea, vomitting but also includes things like hepatitis as well.
Nausea and vomiting. Classical foodborne illness, or gastroenteritis, causes nausea, vomiting, and diarrhea. Fever, headache, and abdominal pain, may also occur. Underlying causes may be viral, bacterial, or rarely amoebic / parasitic. Symptoms usually resolve in 24 hours or less. For dehydrations concerns, severe or persistent symtoms, you should see your doctor.