A1) No/maybe. Not in general. Especially in the case of someone claiming to be God. An angel might announce themselves.
But typically they don't. Sometimes we "entertain angels unawares".
A2) Same as above. No/Maybe. My spirit would bear witness to the evil presence.
And Satan and his minions are liars, so they can't really be trusted or believed anyway.
A demon might claim to be Satan. (or God)
The word 'maybe' in your answer, is an indication of something really wrong.
Eph 6:
10 Finally, be strong in the Lord and in his mighty power.
11Put on the full armor of God, so that you can take your stand against the devil’s schemes.
12For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms.
13Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand.
14Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place,
15and with your feet fitted with the readiness that comes from the gospel of peace.
16In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one.
17Take the helmet of salvation and
the sword of the Spirit, which is the word of God.
In the old times, God had a mission when He sent prophets, messengers, angels, donkey, kings, Judges, seers, mad kings- that time is over and the mission was complete.
It came a time that God Himself came for a mission- that time is gone
In this era, God has already spoken to us through His appointees and was later recorded for our own benefit. There are no more missions- we either receive the word of God as was recorded or reject Him (God) and listen to some one else who may claim to be either an angel of light or satan; it doesn't matter, if it is not from God it is from satan.
The answer to the above questions is a big NO all the way, same as the issue on tongues or healing or miracles. NO NO NO.