To learn if a position has been filled, someone can simply ask the hiring or human resource manager of the company. If the position was posted to an online job board, replying to the ad with an inquiry may result in an answer.
Sometimes, there is no obvious way to tell if a position has been filled before applying to it. While most companies try to delete an employment ad that is no longer relevant from the posting site, some ads remain posted. If the employment opportunity has been posted on an "in-house" job bulletin board, periodically reviewing the board generally alerts the applicant to the status of the job. If the index card has been removed from the bulletin board, chances are good that the job has been already filled. An individual can also search sites such as LinkedIn to see if the company is still hiring, or if the position has been already filled.
Speaking directly to the hiring manager is always prudent because if the existing job has been filled, he can alert the job seeker to other similar jobs that may be still open. If the job seeker emails the hiring manager and gets no response within a reasonable amount of time, he should assume the job has been filled. Spending too much time trying to learn the status of a job opening takes time away from searching for new job opportunities.