Possible posioning of dog

How can you tell if a dog is being posioned? I am very concerned about my dog. He is HW positive and for other reasons is being treated with heartguard plus, instead of the treatment from the vet. He seemed so much better and was back to his old self and no one would have known he was sick and then all of sudden his appetite slowed and and today he has not eaten a bite and seems so restless like he can't get comfortable. How are the symptoms different?

Mickee