So, use this with some caution. This is because the potters were using local cobalt that I believe also contained some iron. Modern late 19th century pieces is often decorated with an annoyingly dark, clear blue. Look for this on ginger jars with big characters on. You should look at, but particularly, FEEL the glaze.
Good pieces is very glossy and smooth to the touch.
This is basically the best rule there is. I think this has something to do with the fact that old porcelain was fired in wood fired kilns. It might also have something to do with the composition of the glaze, but this is too longwinded to go into. Finally I would like to mention one absolutely worthless piece of advise you might find in some older texts on Chinese porcelain, and this is the mentioning of an orange peel effect to the glaze by which you should be able to recognize old pieces. So forget that.