In general, knowing when to use the onyomi or kunyomi will come with increasing familiarity with the spoken language, but there are a few patterns which hold true in most cases. The presence of okurigana (kana attached to the end of the kanji) tends to signal use of the kunyomi. Particularly, this holds true for verbs, in fact, I've yet to encounter a verb stem which is not kunyomi. When more than one kanji in a row is used to express a single concept, the onyomi are typically used. For example: 食べる - TAberu - to eat (kunyomi) 食堂 - SHOkudou - cafeteria - (onyomi) Rules beyond those two become a bit more haphazard. Single kanji words and adjectives are kunyomi more often than not, but there are innumerable exceptions. Numbers tend to be onyomi, except with the counter つ, but the kunyomi for 4 and 7 are often used for stylistic reasons. Some kanji have only an onyomi (such as 気) or kunyomi (such as 緑). Some kanji have multiple readings of either class...

Francis-san is right. These answers may also help. http://www.italki.com/answers/question/80785.htm http://www.italki.com/answers/question/51909.htm