I presume you are referring to Job Postings in regards to obtaining employment. In order to find out what the real requirements are, you rarely will obtain those from HR or Management. However, during your interview(s), you can always ask these questions to the people who interview you:
- What do you think the responsibilities of this position are
- What do you expect from me in this position
You know how you are always asked if you have any questions at the end of the interview? Well, now you have some good candidates.
Short answer -- you find out at the interview. Remember, interviewing works both ways. You do not have to accept any offers if you discover the requirements are nowhere near as accurate in reality as they were in the Job Posting. Good luck!