Psychic readings will never be 100% accurate. It is hard to assess a psychic's accuracy but tests have been done to try and establish how accurate psychics are, with little success. Psychic predictions or readings are usually fairly vague, unsubstantial and hard to verify.
The Delphic oracle in Greece was famous for its cryptic sayings. One technique that psychics use is shotgunning, where they provide a vast amount of information, some of which will be correct, and people tend to remember the bits that are correct and forget the large amount of information that wasn't. Some psychics are adept at extracting information from sitters and making it look as though they provided the information themselves; usually giving the impression they are giving information but actually asking a series of questions and then using the sitter's responses to generate feedback.
How accurate you can expect the information to be also depends on the circumstances. Free psychic horoscopes, or horoscopes in magazines or newspapers, tend to be very generalized and non-specific. This is because they are based on the sun sign alone but a more accurate reading could be given if the time of day and date of birth is given.
Psychic mediums can give surprisingly specific information though, such as names, occupations or significant dates.
It is hard to prove or disprove psychic ability or paranormal powers. The James Randi Educational Foundation has offered a one million dollar reward to anyone who takes up a challenge to prove their psychic abilities but no one so far has been able to win this award.
JB Rhine did a lot of work on proving the existence of the paranormal - none of his experiments obtained very high success rates, merely higher than the rate of chance. The chance rate is the results you would expect to occur by chance. This means that it is not very likely that psychics are able to be completely accurate. If going for a reading, you need to evaluate the information you receive yourself, and how it applies to your life.
Most psychics recommend you record the sitting - this way you can go back to the tape at a later date and check the accuracy of the information in hindsight. Do not expect a completely accurate psychic reading, as circumstances can change shortly after the reading, leading to a different outcome to the one predicted by the psychic. Psychic clairvoyants merely give an assessment of the likely outcome at that time, rather than accurately predicting the future.