There a couple of things worth mentioning here -
1. Communication the most odd thing about the communications claimed to come from gods to people is that they always exactly match the views of the person receiving the message. How often does one hear of someone getting a message from god that goes against how they think? Thus I would expect that, say, Dominic would hacve his views reinforced by these 'messages' - something that would start to make one think that they came not from a god but from Dominic's own sub-conscious.
2. Physical / Spiritual If we are talking about messages being received from a god - one that is spiritual in nature and not physical - then we need to tackle the tough question of how the non-material can affect the material. Like it or not, we are physical beings and to implant any thoughts requires some very skilful 'adjustments' to our brain chemicals and our neuron connections. We need to get down to the mechanics of how this might be possible and how we might test to see if it can be detected. Any ideas on this one Dominic?