It can be hard to discern evil when it first emerges, but after a while this usually becomes very clear.
From the perspective of spiritual warfare, this could be conceptualized as a reflection of the fact that purposive evil is impatient in its destructiveness of the Good (that is indeed one of its hallmarks) - so that evil cannot be very strategic, nor long-termist.
Thus, something of evil intent that is not obvious will (so long as it succeeds) sooner rather than later reveal itself in gratuitous acts of destruction against truth, beauty and virtue.
So, communism. It was at first difficult for many people to recognise this for the evil it is, many people were fooled by its stated intentions - but as there have been dozens of implementations of communism around the world and within most societies, its evil became very obvious (to those with eyes to see) in the systemic dishonesty, destruction of beauty and moral inversions.
'In theory' communism may be imagined to be of good intent, or at least this is debatable - but in practice we know (insofar as we know anything, because anything can be and is denied by evil) that the fruits of communism are evil.
In contrast, Mormonism. When it first emerged it would be hard to say whether it was a good or a bad thing, and on the whole perhaps it looked like a bad thing - in theory, perhaps, it looked considerably worse than communism.
But after about eight generations it is as clear as it ever will be that Mormonism is certainly not evil, and therefore (since nothing is neutral) Mormonism must be on the whole Good. The fruits have shown us what it is.
(Of course, in a fallen world, Good things may become bad, that is indeed the tendency - but I mean up until now.)
But it is strange that so much discourse and debate ignores the fruits of ideas - ignores how things turn-out.
People still talk about atheism, the sexual revolution, or affirmative action, in a totally theoretical way, as if we had to discern and predict their implications - but these are old ideas, and their fruits are as obvious as anything ever is in this world.
We know how these ideas worked out.
By their fruits we should judge them.