le pain, la pomme?
How do you know weather a food is masculine or feminine?
Most of the time, feminine words end in a double consonant + e (mme, tte, nne etc.) or just a plain 'e', although this isn't always the case. Genders are tricky so it's best to just know the word.
There isn't a general rule but you can guess as it's mostly the same in other languages. (there are exceptions) You'll learn it as you progress and read more in french.