I have a question to ask someone who possesses clairvoyant powers which definitely derive from the side of holiness. Who is the best person today to ask? (They also needs to be someone who does not hide what they "see" as many of the Gedolim seem to do.)
1) Most, if not all people who make such claims are charlatans. This may be the case even if they actually believe that they have such powers.
2) There is a severe prohibition against divination. It's also prohibited to ask a diviner. Generally speaking, the prohibition includes any non-scientific method of revealing the future or other hidden information. Simply claiming that it's not from a unholy source is insufficient. For example, the Tsuras Shai wrote about a man who was counseling people with a ouija board. This charlatan would bathe in a mikveh and recite psalms before doing his divination. Despite the man's pretenses of holiness, the Tsuras Shai rejects his activities and prohibits them.
3) See Brachot 3b: "They at once took counsel with Achithofel and consulted the Sanhedrin and questioned the Urim and Tummim" The Malbim (in Eretz Chemda) explains that this teaches the correct order of making decisions. First consult with an expert in nature/science and the ways of the world- that's Achitofel. Then, consult with Torah scholars to ensure that the mundane advice received is permitted. Finally, consult with the prophets. Lesson: Don't run after questionable individuals before exhausting conventional avenues of advice.
4) There is no prophecy today. (In biblical times, prophets were not only consulted to predict the future, but also gave other divinely inspired advice- even for such mundane matters as locating lost objects.)
5) Obviously, you are dealing with an important issue. Please be sure to consult with competent persons- whether they be secular experts or rabbis.
May Hashem grant you a successful outcome to whatever issue you're dealing with.
The Rambam includes divination as the 30th negative commandment in his Sefer Hamitzvot. You can read a translation of the entire work on chabad.org. The Rambam's definition in Sefer HaMitzvot not comprehensive as he only includes one form of 'kosem'. He expands the definition in Mishneh Torah, Laws of Idolatry 11:6. That too can be found in translation on chabad.org
Generally speaking, the prohibition includes any non-scientific method to reveal the future or other hidden information. There are two broad categories. The first involves the individual going into a trance or a sort of altered state of consciousness in which the intellect is suppressed and the imagination is heightened. The second category includes all sort of lotteries or the production of some random 'patterns' that are then interpreted.
The first category includes: 1) The activities of Edgar Cayce- a famous 'psychic' active in the first half of the 20th century. He would enter a trance and then utter 'prophecies'. He would 'diagnose' people's illnesses and suggest 'cures' for them. 2) Scrying- getting psychic visions via staring at a crystal ball. I presume the fabled 'magic mirror' is of the same type. 3) The use of drugs to get spiritual visions. There is evidence (or speculation?) that the priestesses at the Delphic Oracle were intoxicated by natural fumes escaping from the earth and into the temple.
The second category would include:
1) Tarot cards
2) Ouija board (also mentioned by the Malbim)
3) Geomancy: throw pebbles on the ground and interpreting the resulting patterns. Some use a stick to randomly poke holes in the sand and then 'read' the pattern of dots.
4) Making decisions based on the swing of a pendulum.
5) Dowsing for water may also be prohibited.
If the individual does any of these, or similar things- he has violated the commandment against divination. It makes no difference whether he claims a holy source for his speculations. It's the action that is prohibited. If a 'mekubal' does any weird thing to reveal hidden information, it's probably prohibited. (Without going into details, some explain that reading palms is not necessarily included in the prohibition of divination. That doesn't mean that it's not prohibited or inadvisable for other reasons.)
If a 'mekubal' takes money or 'donations', you should be very wary.
Audio lectures for more information: Yisroel Reisman- http://www.mp3shiur.com/download.asp?fn=YD179_1_3%20Hilchos%20Kishif.mp3&prod=2058 Ahron Kahn- http://www.ou.org/torah/article/using_torah_as_a_medication_or_segulah1#.Uh2oVH-RAj8 Herschel Schachter- http://www.yutorah.org/lectures/lecture.cfm/772117/Rabbi_Hershel_Schachter/Segulas,_Superstition,_and_the_Ayin_Hara
Clairvoyance is not Ruach Hakodesh. It is a (pseudo) natural phenomenon.
In fact they are in some ways opposites. The clairvoyant person won't want to hear any details, since that will be bias his perception. When you ask a Baal Ruach Hakodesh he asks you for all details, and will often draw the answer from yourself.
If you do decide to get advice from a holy person, who learns and is known to be pious, you aren't going to hear the future. You are going to follow his advice. When they asked the Urim Vetumim for direction it wasn't a way to divine the future. It was a way to find out what to do.