Because porcelain manufacturers often change their marks over time, the maker's mark usually indicates a general time period in which the piece was made. Some manufacturers even incorporate date codes into their marks. Date codes might consist of letters, numbers or symbols indicating a specific year of manufacture.
There are many online resources devoted to identifying and dating porcelain marks; most with photographic examples. Antique-marks.com is a comprehensive site that covers porcelain from many regions and time periods. Thepotteries.org focuses on English manufacturers but also contains useful general information on dating clues found in makers' marks.
If you know the manufacturer or region where a piece was made, you may be able to find even more detailed information in resources specializing in a particular type of porcelain. Pay special attention to any notes on reproductions or fakes, because especially valuable or popular pieces were often imitated or even copied by other companies.