How Can You Tell If Someone Is Poisoning Your 6

How can you tell if someone is poisoning your 6 year old child? My daughter finally came out about something that her dad and uncle did to her and legal actions are being taken. Well my daughter was fine and at my sister's house and her nana, which is her dad's mom went pick her up, when I found out I went the following morning and pick her up immediately and her nana was surprised and quick to tell me that she was sick throwing up and having diarrhea because of some blueberries she ate, but after I got her with me she only threw up once and it was clear like foam and that was it, I gave her some medicine with a cool towel and she was back to normal...is it possible that she could have been poisoned? And what would she have been poisoned with?