On pots, you will also see the blue coloration where the spout joins the body and where the handle terminals connect to the body. Some times , you will see a bluish tint on some random spots. This is usually caused by extra thick glazing in those areas. The only problem is that some porcelain pieces also have a bluish glaze. You have to eliminate porcelain as a possibility.
Porcelain is translucent when held up to a light.
Pottery is not. If it is not translucent, then you have pottery. Creamware will have a glaze that is clear, through a yellowish tint, and then possibly a greenish, grey tint. These characteristics are easily seen in the crevices where the glaze puddles. Another characteristic of creamware is that if you look straight down, you can often see an irridescence or multicolors as in a rainbow, although this is not too common.