Me too, In my experience, evil spirits seem capable of every lie including pretending to believe the tenets of Christianity.
I think St. John's passage is possibly pointing to a group of people and not literally spirits but people who may be influenced by an unGodly spirit.
For years demons tried to fool me with every permutation of leaving 1 word out of "i confess Jesus Christ is come in the flesh"
(Jesus is the person, Christ is the office so to speak. like saying i confess obama is come in the flesh, but i'm a birther and he isn't the president because he was born in kenya" so i won't say the president obama, but i will admit obama was elected. )
and so for what its worth, in 12 years of being on the internet in religious forums, when this topic comes up, i have never met someone who could confidently tell me, a demon told them "i confess Jesus Christ is come in the flesh"
Secondarily, spirits from God, would instantly speak those words before i even finished replying back to them, demanding the test of spirits from 1 john 4:2. meaning they knew what i was going to say before i said it. demons don't have that capacity.
Demons on the other hand, in general.. will always get angry the moment you question them, or they will respond in an unrighteous manner outside the usual, kind gentile responses you'll get from Jesus.
I had a friend years ago who told me she would make the spirits recite the nicean creed, as a test of spirits. the moment she told me this, a demon started reciting the creed to me, and i told it to shut up. the only way i knew it was a demon was, the context, the mocking voice print, etc.
As far as telling the difference between an unclean one and a clean one.. you can't tell by voice print alone. sometimes you can tell by the emotions, (anger wrath, hate, jealousy).. they can't fake the presence of a loving God.
I think that a lot of people just listen to what feels good to them. and, strangely i think that can work as long as you have time to ponder the message and pray about it, read the bible, and ask for confirmation.
Another friend of mine told me once, something told her to drive through a red light. she was getting close to actually doing it, and her 9 yr old son yelled, "mom no!" --Turns out the 10 yr old heard the demon speak to his mom, and knew it was a demon, while the mom was confused.
which brings another interesting memory, back in 2012 when i met them, i spiritually sort of knew the son was saved but not her daughter. a few years later, 2014 maybe, in a vision at like 9 am i saw the mother and the son, holding hands looking up at God with joy in their hearts. i didn't see the daughter. while pondering the vision, half an hour later she texted me for the first time in like 6 months at that point, and asks "does God ever show you what he's going to do in a persons life before he does it?" and i'm just like dumbfounded. and i reply "no but i saw you half an hour ago, does that count? and she just replies "not what God is doing". maybe i should have just told her plainly what i saw. but, i'm not sure she would be any less creeped out.