When a person claims they are seeing the angel Gabriel giving them a message from Allah, there are a number of possibilities:
The experience is real.
This person is sincere, but experiencing a mental illness.
This person is manipulating the gullibility of simple minded people that can't think for themselves.
Other possibilities
How are Muslims to tell the difference between these possibilities, and therefore be guided towards a true prophet of Allah?
Even if his claim is true, it does not mean that he is a prophet. There will be no prophet after Prophet Muhammad (May Allah's blessings and peace be upon him), anyone who claims to be a prophet is either a liar or a crazy person.
There will be no prophet after me.
لاَ نَبِيَّ بَعْدِي
Even though this is a hypothetical question, the simple answer is that a Muslim would weigh the message and see whether it appealed to their senses, seeking guidance from Allah.
This is no different to a non-Muslim accepting Islam today, or a Muslim continuing to believe in Islam.
As far as I know, all prophets confirmed the prophets before them and introduced the ones to come after them. Muhammad (pbuh), being unique in that there wasn't going to be a prophet after him, confirmed the prophets before him like other prophets but unlike other prophets, he informed us that there would be no further prophets after him.
He prophesied the return of Jesus (as), but as a follower, rather than again as a prophet.
This is a question I do personally grapple with as well, because Muslims of a certain period of time will be faced with similar questions, i.e., how do we recognize Dajjal? How do we recognize Jesus (as)? How do we recognize the Mahdi?
While none of them are going to be prophets, I think in spirit, the problem is similar and the answer lies in http://quran.com/2/26.
Indeed, Allah is not timid to present an example - that of a mosquito or what is smaller than it. And those who have believed know that it is the truth from their Lord. But as for those who disbelieve, they say, "What did Allah intend by this as an example?" He misleads many thereby and guides many thereby. And He misleads not except the defiantly disobedient,
Short version: keep a seeking, open mind.
If we are talking about our beliefs - as Muslims who believe in Qur'an - there is no prophet after Prophet Muhammad (PBUH):
مَّا كَانَ مُحَمَّدٌ أَبَا أَحَدٍ مِّن رِّجَالِكُمْ وَلَكِن رَّسُولَ اللَّهِ وَخَاتَمَ النَّبِيِّينَ
Muhammad is not the father of [any] one of your men, but [he is] the Messenger of Allah and last of the prophets.
But if we are talking to the other people who don't believe in Qur'an, we can say: OK, so what's the new messages of God?
It remains to compare the messages Prophet Muhammad (PBUH) has brought from God - without being educated, without having internet, without being grown in scientific community - with the new messages somebody claims are from God.
He should be introduced by the earlier prophets, like the prophet Muhammad --peace be upon him-- was introduced in Torah and Injeel at the time (maybe the current translations of the books do not contain these announcements, I don't know). We even know people in history of Islam who knew the prophet of Islam to be a prophet only based on what they were told by the Jew scholars, although they never become believers as well. The last proved prophet but has announced that no new prophet would be sent to guide people after him --that Jesus peace be upon him will return before the Resurrection doesn't contradict with this announcement obviously!-- so that any one who would claim to be sent as prophet would simply be a liar.
Prophets should have miracles as another proof for their prophecy, to show they are the prophet announced by the previous prophets.
PS. About Miracles. Everywhere in Quran miracles are referred to as Signs (آیات), Clarifications (بینات) or Clarifying Signs (آیات البینات or آیات المبینات), but the signs that according to [27:93] people would know them when is show to them, thus if they don't believe then they would be Kaafir (denier). However, there are now people who can do many of such historical miracles, like in Sufism and Erfaan, even doing some of those deeds need not a person to be a strong believer, an atheist like those in India can do many odd jobs, but these are not miracles to prove their prophecy. Miracles of Moses was for he time of Moses, miracles of Jesus was miracle at the time, peace be upon both of them. Ad-Dajjal can kill and rise like Jesus rose his friend, but Ad-Dajjal is not even a believer. People will think he is God like Christians now think Jesus is God, but imams have thought us the benchmarks not to get lost. The miracle for the present time and upto the Day would only be Quran, no one till then can ever bring a book like Quran. That the previous miracles are no longer miracles is due to improvement of knowledge and knowledge completed with the last prophet, provided to us through Quran and the Prophet's house-hold.
At first I should separate the question to two parts:
What should I do about that person or about that event?
In Islam we should just concern about our prophet and those in authority among us -as said in Surah An-Nisa: 59-. So I will pass up.
What should I do about that message?
In Islam we should send that message to those in authority among us -as said in Surah An-Nisa: 83-. So I will get a confirmation about that.
HTH
