The agent of a passive construction is in ablative, and human agents also come with the preposition a/ab. For example, Marcus a Gnaeo occisus est but Marcus sica occisus est. But which agents should be considered human?
Many cases are clear, but there are also less obvious situations. For example, are gods, dolls, anthropomorphic robots and pets with human-like status human? My Latin grammar gives the example A Deo mundus creatus est, implying that the Christian god can be viewed as human — without implying that it would always be viewed so.
Can you give some guidelines for choosing which agents to consider human and which ones non-human? I would prefer the guidelines to be backed by classical usage or a reliable grammar book.