In book V, chapter 10 of De Mysteriis, Iamblichus gives us an elegant and straightforward method for distinguishing genuine Gods and Daimones from impostures.
We observe that the Demiurge provides us lower creatures with everything we need for our sustenance. So, why should he fail to do so, for the daimones? If their sustenance were adventitious, dependent on us (such that our neglecting them would somehow harm or disequilibriate them), then they would be inferior to us, and we would be superior to them, which is absurd.
We can conclude that any being which depends upon an offering is an inferior being, and not the God or Daimon to whom sacrifice is properly given.
As Iamblichus himself explains, “Each thing derives its nurture and fulfillment from that to which it owes its generation.”