The topic is raised at William Wildblood's blog - in relation to his own, early adult, experiences with channelled communications from higher spiritual beings (not angels, or not necessarily angels).
This provoked me to think, and elicited the following comment - here expanded:
The demand to know how we may distinguish for sure between good and evil spirits, or between angels and demons, or white and black magic is one of those situations in which people seem to be asking for something which cannot be had in mortal life - absolute-infallible-objective-eternal certainty of knowledge.
Such is not available to us. We cannot - in that hard sense - 100% determine the difference between (say) angels and demons. After all, we might currently (at this moment) be insane, delirious or demented - or asleep and not knowing it.
But there is nothing specific to that class of discernment. It is qualitatively the same discernment we need to make between someone who loves us and someone who loathes us and is manipulatively pretending to love us - yet we must make such a discernment to marry. Or between a saint and an Antichrist - yet we must make such discernments to live consciously.
In this mortal life, we are not made (nor is the world made) to be omniscient; but instead called upon to make a truthful and honest evaluation, and to learn from the consequences. That is, Mortal life is for experiencing and learning - in ways that benefit us in the coming (if chosen) eternal, postmortal, resurrected, Heavenly life - we are not supposed to 'be perfect'.
(If that was the intention, we would have to conclude that God-the-creator has badly-designed our-selves and this world.)
I think what covertly (or explicitly) lies behind the kind of question 'Are 'The Masters' demons?' is the assumption that there is a 'safe' way of being a Christian, where we do not need to make such discernments.
The idea that (for instance) by reading and obeying scripture 'literally' we will all be saved; without having to hazard the 'risks' of dealing with spiritual, supernatural, 'esoteric' phenomena...
By contrast, I see no safe way to be a Christian, there are hazards in every direction. For instance, materialistic legalism is a major hazard of supernatural-avoiding, anti-spiritual Bible-based Christianity.
There is no safe path, there is not meant to be a safe path; and there is no path which fits the needs of everybody (we each have different weaknesses, and strengths). Your actual life and my actual life just-are-unique, and deliberately so; because our spiritual needs are unique.
We are meant to live without certainty in order that we may learn through living.
Thus, life is irreplaceable.