As an aside, I’d like to know what sort of background you have with religion. The reason I ask is because I get the impression that you have some fundamental misunderstandings not just about Catholicism but religion in general.
The difference between discerning the morality of two strangers and discerning the difference between God and Satan is something fundamental. Discerning two strangers on the street requires the limited capacity of one’s own intellect. Whereas discerning between God and Satan does not rely solely on one’s own intellect. It relies mostly on work that has not been done by me, but by all those who have come before me. This principle is not much different than that of building upon scientific discoveries of the past. This is why Catholics have the Church. It’s our “body” of work on the discernment of morality…I have given you a link to part of it - Theology of the Body - have you read it yet?
But the reasoning remains for the individual as well as the reasoning that has been done by past generations. But I guess I could give you some of the more basic litmus tests that Catholics use. The most basic is judging by the fruit that comes from that which is being judged. There are others but they require more than what a forum is capable of giving effectively.
So, I hope you see the difference. It’s one of the reasons Catholics don’t encourage Christians to form their own churches, because you sort of lose that body of work that has been built upon for 2K years in favor of your own interpretation of anything from scripture to morality. There is no need to re-invent the wheel every generation.