In Middle-earth, there are various ways of seeing things far away both in distance and in time – the palantíri and Galadriel’s Mirror, to name two. But whilst all this may suggest to some that Tolkien was promoting such activity, which resembles fortunetelling and other such occult activities, in fact it is quite the opposite. J.R.R. Tolkien in The Lord of the Rings sets an excellent example in the field of magic for other fantasy writers.
Before I begin my discussion of the palantíri, it would probably be helpful to know exactly what they are. Palantír means “that-which- sees-far- away” and they were used by the Kings of Men – Isildur and Elendil, for example – to keep in contact with the rest of their kingdoms. Originally there were seven, scattered through various locations in Middle-earth. The ones still existing in Middle-earth at the time of LOTR were those of Minas Tirith, Minas Ithil, and Orthanc. Sauron captured the second when he took hold of Minas Ithil in the Second Age, and from then on, use of the other palantíri was even more dangerous than it ever was.
The use of a palantír is fraught with danger for those who do not have a right to use them; during the War of the Ring the only person who had this right was Aragorn. When Denethor uses the Stone of Minas Tirith, he sees the Black Fleet approaching, and as the palantír does not show everything but only some things, he doesn’t realise that Aragorn by that point has actually taken over the fleet. But this alone should not have driven him to suicide: it is clear later on that he has in fact seen Frodo, captured by Sauron. Tolkien does not explicitly say so, but later on Denethor says to Pippin,
“Comfort me not with wizards!….The fool’s hope has failed. The Enemy has found it, and now his power waxes…” 
Denethor is oblivious to the fact that Sam, thinking Frodo to be dead, took the Ring from Frodo in Cirith Ungol, and therefore although Sauron may hold the Ring-bearer prisoner he does not have the Ring. That not everything is visible in the Seeing Stones and therefore appearances can be dangerously deceiving, should be taken as a warning against trying to see the future.
Galadriel’s Mirror is the other main form of vision in The Lord of the Rings. However, Galadriel herself explains the dangers of the Mirror to Frodo and Sam.
“[The Mirror] shows things that were, and things that are, and things that may yet be. But which it is that he sees, even the wisest cannot tell…. The Mirror shows many things, and not all have yet come to pass. Some never come to be, unless those that behold the visions turn aside from their path to prevent them. The Mirror is dangerous as a guide of deeds” (emphasis mine). 
Divination, besides being black magic, does not show the entire truth, and therefore it’s dangerous to use fortunetelling as a guide for your deeds. Who knows what Emperor Frederick II may have done if he had not been told that he would die under roses! “Perilous to us all are the devices of an art deeper than we possess ourselves,” says Tolkien in The Two Towers.
Although this sort of thing may be permissible in some fantasy where it is not acceptable on earth, it is still necessary to be very careful with the concept, because regardless of the author’s intent, people are easily influenced by books they read. Use of the palantíri and the Mirror of Galadriel is very clearly dangerous from the text of LOTR, and the last thing the Professor was encouraging is use of fortunetelling methods.
 Tolkien, J.R.R. The Return of the King (London: Unwin-Hyman, 1955), 97.
 Ibid., The Fellowship of the Ring, 377-8.
 Ibid., The Two Towers, 203.