The date of publication is not always easy to determine, particularly for older works. For a work published in the last century or so, the date should be listed on the title page, along with the name and location of the publisher. If no date is given, try looking for a copyright notice; if one is present, the year listed will be the year of first publication. If you still can't find a date, consult bibliographies and other reference sources to see if someone has already looked into this question and found an answer.