The book of Daniel is unique in the bible prophetic books because it describes its predictions in a luxury of details. The fulfilment of Daniel prophecy cannot be anything like those hammerd by Mathew to dubiously claim that Jesus fulfilled old testament prophesies. There is no room for ambiguity in Daniel. So the only way a skeptic can dismiss Daniel is to claim that its fulfilled predictions are vaticinium ex eventu ( written after the events). Indead the claim that 'it was all an hoax' is the general default of skeptics when the evidence is too clear to explain it away using other ideas e.g. 'hallucinations' or 'misinterpretation'.
But do the idea that Daniel is vaticinium ex- eventu conduct electricity? As we will see, it doesn't. On the other hand, it is hard to say thay Daniel is an infallible works of some omniscient being. Indead Daniel predicted empires and emperors with astonishing accuracy, down from Persia to Alexander the great all the way to culminate in Antiochus Epiphane. However, it suddenly begun to make false predictions! It, for instance, begins to insinuate that resurrection and kingdom of God will follow immediately after the era of Antiochus! It is this prediction that is at the heart of confusion around Daniel, specifically, the mistaken idea that Daniel is talking of the future end times.
The major problem with the idea that Daniel was vaticinium ex eventu is explaining how it was unanimously accepted by Jews. People often make the erroneous insinuation that ancient people were never critical. This is not the case as is evidenced by existence of many mutually exclusive sects in ancient Judaism. This means that each of the sects were critical of the other. We have Zealots, Essenes, Gnostics, Pharisees, Saddusees, Christians etc. Particularly interesting is the Saddusees case who did not believe in resurrection. Were it true that Daniel was vaticinium ex eventu, Saducees would have certainly pointed it out as Daniel clearly teaches a future resurrection. Take Jesus's case, for instance. We know that the belief that Jesus resurrection was not unanimous because the book of Mark mentions how sceptic of the day rationalized the empty tomb.
We also know that Jews did not include all books in the canon. They did not include the book of Enoch written at about the same time when Daniel is supposedly written. The reason is that Enoch was apocryphal. However if Daniel was vaticinium ex eventu, it too would be apocryphal! Thus we see that Jews did judge whether or not a book was a vaticinium ex eventu before they accepted that it was an inspired word of God. So it is impossible that a book written just the other day would be accepted into the cannon without a split in Judaism based on the authenticity of that particular book. For instance a future historian will know that the teachings of Elen G White were not part of the earlier Christian teachings even if he is not told so. He will tell this from the fact that Christianity splits on the basis of those teachings alone, so that we have a new sect that differs from the other in that it believe in the authenticity of the works of Elen G White. Similarly, the sudden introduction of Daniel would not possibly convince every Jew. Therefore a sect that revolves around the authenticity of prophet Daniel would branch off Judaism just as Christianity branches off based on authenticity if Gospels, and Islam does the same based on authenticity of Quran.
The book of Daniel is theorized to have been written at the periods around the Hanukkah wars. This adds to the problems as another book called 'Meccabees' was written at around that time. However, the authenticity of meccabees is questioned among Jews. This means that at around that time, it was not easy to introduce new books into the canon of Jewish scriptures. Furthermore meccabees was a pro- hasmonean. This means that the triumphant kingdom had no authority to hammer in a book and declare it to be the 'word of God'. It it had such, it would have declared that meccabees was so. So at best, if Daniel was vaticinium ex eventu, it would have been like mecabes, found in some compilations but never found in others.
Also, the book of Meccabes mensions Daniel and mensions conflicts and even changes and corruptions in Jewish priesthoods. But it never mensions anything about finding an ancient book that foretold the events that meccabees was zeroing in. Were it true that Daniel was vaticinium ex eventu, the writter of mecabees (more of an historian ) would try to rationalize its absence by mensioning that a book of Daniel, foretelling the events were found. Instead, he mensions Daniel as if he were already a well known ancient hero in Israel, alongside David, Joseph and other heroes.