In most cases, the distinction between forms of the genitive and accusative case presents no difficulties: only need to pay attention to the case endings.
If the end of the two forms of the same, you need to act on the following algorithm.
If before you inanimate noun,then you should ask a question in this way. Nouns in the genitive answer the question "what?" And combined with the word "no." Nouns answer the question "what?" In the accusative, and combined with the word "see." For example: I wear (what?) Coat - accusative, go without (what?) Coat - genitive.
If before you animate nounmasculine II declination, you should substitute the word I any inducement and look at its end. For example: wild boar shot = shot fox (y ending - accusative), wild boar scared = frightened fox (end s - genitive).
If before you animate noun,standing in the plural, it should be replaced by an inanimate nouns in the same form. For example: I like people, I like (what?) The letter - the accusative case. I like the sincerity of the people, I love the sincerity of (what?) Letters - genitive.