Very regular, very complicated. Newer cards use a much bolder font. The period is a circle raised above the baseline of the text.
The period is still a circle, but it's at the baseline of the text, like in English. Japanese also has several different kinds of characters, some like Chinese (kanji), some very simple and linear ones used for foreign words (katakana), and some very curly (hiragana). The hiragana (such as the last two characters in the above text) are very much unlike the Chinese and Korean characters.
In Korean, the period is just a dot. It's also the only of the three to put spaces between words. The characters are also quite distinctive, each one made up of simpler pieces.
The main way to tell if you have a Simplified Chinese card or not is by the font. The text of the card is written in a brush-like font, whereas traditional Chinese cards use a constant-thickness, boxier font. Here's a comparison:
Simplified: 
Traditional: 
You can also look at the chart of card types, but this will only work for some card types and if you have a good eye for detail.
Update! Well, Wizards just wants to keep you guessing. Newer Chinese cards now have a round period on the bottom of the card, like Japanese. In addition, the new Simplified Chinese cards use the boxier font instead of the brush-style font. I'll have to scan some more examples some day...
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