The sheep-goat effect was a phenomenon discovered by psychologist, Gertrude Schmeidler, in the 1940s in New York. Subjects who believed they were going to do well scored highly in tests and sceptics scored well below the accepted rate of chance (psi-missing). In practical terms, how accurate your psychic is could depend on factors such as was sort of mood they are in and how well you click with that person.
Going into a reading with an open mind and feeling relaxed gives the best chance of a good reading with an accurate psychic. However no psychic is going to be 100% accurate all the time. That is not possible and everyone has off days.
Most psychic predictions tend to be fairly vague and non-specific, for instance the horoscopes in tabloid newspapers. However, astrologers can give a more specific reading if they know the time of day and the date of your birth. If you have a good idea of what issues you want addressed in your reading, it allows your psychic to help you in specific areas of your life, rather than give you a general reading which may not cover the things you want to discuss.
It is easy to claim to be an accurate psychic but hard to prove or disprove. Some so-called psychics rely on tricks to amaze their audience. One trick to look for is cold reading. This is when the psychic will ask you questions or give you information and assess your reaction to it to work out whether that information was relevant to you or not. Afterwards people tend to remember the correct insights and forget the inaccurate suggestions the psychic has made. This relies on the subject's desire to believe in the psychic's powers.
Ask the psychic whether you can record the session. Most will let you do this, allowing you to refer back to the reading later on. Sometimes psychics say things that don't seem relevant at the time, but may make sense later on.