Someome giving my baby cereal?

Nastasia

Would I be able to tell based on my babys poop if someone gives him cereal? He is 3 months and on breastmilk only. I am just paranoid as grandma is watching him and suggested before giving cereal would help him sleep better as this what she did 30 years ago. I just wanted to see if someome knows if poop texture changes before I "accused " someone of giving him anything he's not supposed to have yet. Thank you for any responses in advance!!