German Johann Friedrich Boettcher discovered the secret for making porcelain in 1709, and soon thereafter founded the Royal Meissen Factory. Within forty years, there were eight major German porcelain manufacturers, and by the turn of the 19th century, more than 20 additional companies had been founded. All of these companies--as well as modern companies--can be identified by marks placed on the undersides of their pieces.
Look at the underside of the porcelain piece in question for a mark. Some early marks were simply symbols, and did not state the name of the company or its city of origin. However, later marks generally include the company's initials or even full name, as well as its location.
Search for the mark in porcelain reference books or on the Internet. Two good sites are Porcelain Marks and More and Ceramic Link. These sites have German porcelain makers listed alphabetically and by city, and contain pictures of hundreds of marks.
Remember that marks for particular companies changed throughout the years. For instance, the Ludwigsburg Factory, established in 1763, changed its mark every few years throughout the 18th century.
Once you locate the mark in reference materials or on line, you will find the corresponding company name and location.
Research the particular qualities of the porcelain piece in question in order to date it more accurately. For example, a particular mark will generally date a piece to within a few decades, but you may be able to narrow the time period by discovering that a certain factory only made a certain type or color of porcelain during a shorter interval.
If you have an early piece of German porcelain, the mark of which does not contain the manufacturer's name or location, you may have to look through dozens or even hundreds of marks before you find it.