[shooon]

Only Japan uses Hiragana and Katakana.
So if there are Hiragana or Katakana letters, it must be Japanese.
Korea uses almost only Hangeul.
So if there are Hangeul letters(+ few Kanjis), it must be Korean.

But there is another way to know which lanuage is used.
It is to distinguish Kanji(Chinese character).

There are 3(4) types of Kanji in the world.
Kanji in China, Taiwan(and Hongkong), Japan(, and Korea).

At worst, one kanji is written in three different ways.
For example,

China:乐
Japan:楽
Taiwan：樂

At best, the three countries use the same letter.
教,育,改,and 革 are examples (as kirakiraさん said above.)

China invented new kanji style in 1960s.
In order to make writing kanji easier and faster.
On the other hand, Taiwan has been using the most traditional (and complicated) kanji.
Japan is halfway.Some kanjis were changed and others were not.
Korea usually doesn't use kanji today. But when used, their kanjis are similar to Japanese or Taiwanese kanjis.

Other examples:
China:对
Japan:対
Taiwan:對

China:卖
Japan:売
Taiwan:賣

So if you can(?) tell which kanji is used, you can tell which language is used.