Angels have no "Ego" so they have no reason to manifest. As with any Tradition as you start and progress further down your chosen path, one can expect visitors from either side.
The "Bad" ones come out of curiousity and try to get you in an uproar (This is How They Feed) I for myself remain neautral as they get bored and go away. Should you challenge them you may or may not win, he will in turn bring his boss, and the cycle repeats until you are way above your head.
Angels "Good One's" appear when "Good Work" is done as they are attracted to the vibrations that emit from the individuals.
If for example "Healing" is being done then one could problaby surmise they are in attendance. They have no reason to attend unless asked and even then the vibration must be conducted of the "Light". Sometimes they will purify the area so as to bring more of them.
There are certain Invocations that will call down both sides, I can't imagine why anyone would call something that thinks humanity is a bunch of idiots, and will do everything within it's power to deceive the individual.
Your Thoughts carry much power guard them well, "As A Man Or Woman Thinketh So Shall It Be"!
Learning anything is only a process of remembering that which we have surpressed. You can read countless books, attend countless seminars, and in the end you have only to ask yourself What is "Truth" to you?
I never just give anyone an answer, I let them find their own way and by doing so, they claim the power as their own.
In turn their Aura becomes a little brighter. and as a result so does the Earth's.
peace
Ublisi