The easiest way to determine masculine and feminine nouns is to look at the pronoun before it. If the noun is masculine, there will be a "le" or "un" and if it's feminine there will be a "la" or "une", but if it's plural, there will be a "les" or "des", or of it starts with a vowel, there could be a "l' ".
When the noun is plural or starts with a vowel, it is a lot harder to tell, so you have to look at the word. In general (there are always exceptions), if a word ends with -que, -ée, or -elle, it is feminine.
Just a note, there are a lot more masculine words than feminine, so if you really can't figure out which it is, I would defer to the masculine.
Hope this helps :)
