Here's why:
* The ending "e": Many feminine nouns in French end in "-e" (though not all).
* The definite article: The definite article "la" (the) is used before feminine nouns. So, "la campagne" (the countryside).
You can also use this trick: If you add the word "la" (the) before the word, it will sound natural if it's feminine and awkward if it's masculine.