Valuing fine porcelain is usually best left to the professionals but there are a number of things you can look for to try and date and value a piece yourself. Look carefully at your piece and learn to read the signs.
Look at the mark or stamp on the porcelain. Imports from the late 1800s onwards displayed the country mark on the product, so a marking of the country of origin shows that the piece can't be older than this period. The more detailed the marking usually shows that it was made later. Look also for the manufacturer's stamp, which you can then look up to see how rare the item is.
Assess the condition of the porcelain by looking carefully for scratches or chips. Because of the fragile nature of the material, any breakages or signs of previous repair may greatly decrease the value but again this depends on the edition and availability of the piece in question.
Hold your porcelain and feel the weight of it. A general rule is that heavier pieces were actually made later than lighter, more fragile pieces but this will also depend on the style. There are many telltale signs of style from the fashions of the place and period such as spherical teapots from the seventeenth and eighteenth centuries.
Search antique catalogs or online information (see Resources) for similar products or manufacturers. This is not always a fail-safe technique because there are so many imitation products, but this will give you information to match the style of the piece to a certain era or place.
Take your porcelain piece to a professional appraiser. If you need to know the value of your fine porcelain before selling or to include it on an insurance policy, you need to have a trained eye look it over. With so many exceptional fakes on the market, get a second opinion before committing yourself to anything.