How do we know how old a cup is? One of the interesting things about this hobby is that these cups are mostly tied to history - personal, unit, or political. There are cups that are tied to individual histories - someone did this or that. There are ones that are tied to particular units - the Manchukuo Railroad units, the Taiwan regiments, individual warships, etc. Then there are the ones that are dated to events - Sino-Japanese War, Russo-Japanese War, WW1, WW2, etc.
Among these, only the last kind are very easily datable. Usually it says right on the cup what it's for, like this Taiwan Conquest cup for the first Sino-Japanese war. The Japanese sent some units over to Taiwan to take it over by force after it was ceded to them in the Treaty of Shimonoseki. So with cups like this you can date them pretty easily.
Then there are cups that don't name any dates but you can put a pretty close date on the cup because of particular images or themes. For example this Mikasa cup:
You're not going to get a cup with the Mikasa on it past maybe 1920 at the very latest, and chances are it's a cup that was made around the time of the Russo-Japanese War.
Beyond that though, you have to rely on the style of the items. As Rich has mentioned on his site before, generally you have two main styles - the sakazuki, which are the flat, wide cups, and the smaller, round ochoko. Sakazuki are often sold in three stacking cup style, with 3-3.25" being a common size. Ochoko are almost always 2" across, although some are a bit bigger and flatter at 2.25". The dividing line between sakazuki and ochoko is about the same time as the transition from Taisho to Showa - around mid-20s, basically. A lot of Showa enthronement cups are in the style of the ochoko, whereas you almost never find one for the Taisho enthronement that is an ochoko. Sakazuki basically don't exist for the later periods. A typical ochoko is one shaped like this:
There's one exception to this though - lacquered cups. As opposed to the porcelain cups, lacquered cups continue to be made in the sakazuki style. There is probably a practical reason for this - ochoko are harder to make when you have to carve down a piece of wood to get the desired shape of an ochoko. Also, lacquered cups were more expensive and more formal, and usually were reserved for more important things - events having to do with the emperor, major commemorative events, stuff from officers of various sort. There's a clear correlation between the rank of people getting awards and the proportion of them being in lacquered-ware. The higher rank it is the more likely it is to be made of lacquered wood. They were more valuable, even if in many cases the porcelain cups have more interesting images. An example of a rare, late period sakazuki in lacquered wood is this cup that commemorates the capture of Singapore.
There are similar style hints for bottles too. Older bottles tend to be the white ones that are 6" tall and have straighter sides. They are usually lightly decorated - a pair of flags, some flowers, that sort of thing. Once in a while you get a nice one like this bottle:
Later bottles, coinciding with the sakazuki to ochoko transition, tend to be more decorated. You start seeing those blue and brown bottles with embossed background and all kinds of stuff on them. You have these bottles with tanks on them. You have bottles with slogans embossed on them. The decorations really took off after the 1920s. Like this Manchukuo bottle with a nice wreath - you'll never seen bottles with this kind of decoration prior to about 1930.
There are of course potential exceptions, but usually these things work in reverse - you might see a later period sakazuki, but the only ochoko I've seen that definitely date to earlier than the 1920s would be those special photo-transferred ones with Russo-Japanese war admirals and generals. Like this one of General Oshima:
I hope this helps in providing some visuals to the general rule of thumb of dating these items.