Instructions
Look at the base. Each cup should be marked "Made in Japan," usually in iron-red. There may also be a maker's mark as well. Noritake is the most collectible maker.
Examine the porcelain body of the tea cup. It should be eggshell thin and practically weightless, while also having a matte, chalky appearance.
Consider the shape next. Japanese tea cups were usually made in conservative Western styles, the most common being a low, rounded bowl with a small curved handle. Any unusual, angular or forward-looking shapes would suggest another country of origin.
Check the decoration. In many cases this drew on oriental themes such as cranes and reed-beds, but it is just as common to see entirely European subjects portrayed. The common denominator is that the decoration was traditional -- almost old-fashioned -- and picturesque in a chocolate-box way. For instance, you're unlikely to see Japanese tea cups in a bold Art Deco style.