By the early years of the 20th century, Japan was making a wide range of ceramics for export, ranging from exclusive Satsuma art wares to more utilitarian pieces for mass consumption. In particular, the 1920s and '30s saw Japanese manufacturers produce quantities of tea sets, which you can easily identify.
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 collectable maker.
Examine the porcelain body of the tea cup. It should be eggshell thin and practically weightless, while also having a matt, 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.
Japanese tea sets were mass-produced, so if you're interested in purchasing one, dig your heels in and haggle until the price is right for you, because another opportunity to buy will come along soon enough.
Most of these cups were put away in cabinets and have thus survived unblemished, so don't buy any that are in less than perfect condition.