There it is. The Dawoodi Bohra belief - The Dai will guide his followers and proclaim the True Imam.
You had better be certain of what you are saying because my understanding is that no one but Imam will determine how he will be recognized. However, please do point out a Fatimi/Tayyibi book that endorses your belief.
Returning to the subject of this thread, let me summarize what most on this thread are familiar with. Remember, it is you who raised this highly speculative question which has the merit of being entertaining but not much else. I do not think that any Imam is coming. Nevertheless, here we go.
You have presumably read the Risala No. 24 of Sayedna Taher Saifuddin called 'dhaat al-barakaat'. In this he describes a method by which the Imam, presumably Imam al-Tayyib, made himself appear to one person outside his immediate entourage. That person was a Maghribi Sunni trader, a follower of the Maliki madhab. At that time, Imam resided in a valley in far West of North Africa surrounded by mountains and his residence was not easily accessible.
Imam had left 3 crystal glasses with his Dai in Cairo with the instruction that if the Dai desired to know the whereabouts of his Imam, he was to sell one of the crystals. The crystal would then find its way to the Imam. The crystal was bought by the above-mentioned Maghribi trader. The trader found it difficult to re-sell the crystal because it was extremely expensive. But the trader encountered an emissary of the Imam who took him to the Imam. Before the trader could say anything about the crystal, Imam described it to him displaying his 'hidden' knowledge. He then gave instructions and money to the trader to buy the remaining crystals from the Dai. That is how the Dai came to know the whereabouts of his master.
We can conclude from above that Imam does not explicitly communicate with the Dai but leaves clues by which he might be identified. Thus Imam alone will identify and verify himself. He may or may not use the Dai. In this instance a Sunni trader pointed out the Imam to the Dai. However, the entire recognition game was set up by the Imam himself.
However, let us consider how the recognition of the 'true' Imam has historically been beset with problems to the extent that even his hujjat in Yemen, Hurrat al-Malika, could not be certain whether he was dead or alive.
Following the assassination of Imam Aamir, his cousin Abdul Majid, claimed Fatimid Imamat for himself. Abdul Majid justified his accession to Imamat by claiming nass from Imam Aamir. He let the story out that al-Tayyib's birth was expected but he was not born. Later he let it be known that either his pregnant mother had died or that al-Tayyib had died in infancy.
Some scholars say that Hurratul Malika had for a short time accepted Abdul Majid as the Imam and appointed Ibrahim al-Hamidi as the Chief Dai. She later changed her mind and upheld the claim of al-Tayyib and appointed Dhuaib bin Musa as the Chief Dai. Following Hurratul Malika's death, Ibrahim al-Hamidi pledged allegiance to Dhuaib bin Musa and succeeded him as 2nd Chief Dai.
Thus it appears that we are to rely on miracles to confirm identity of Imam. If that is not available, any contender for Imamat will have to produce proof that he is legitimate descendant of all previous Imams upon whom nass was pronounced. Since that would be quite a task, we can safely say that no Imam is coming and that the Dai will continue to reign for ever and end. The Imam verification ploy is a futile, even though an entertaining, diversion.