Masculine and Feminine Forms of the Same Word
How do you know if a word has both masculine and feminine forms?
Examples: chatte & chat, noire & noir, une & un
How do you know if feminine words like tomate, viande, soupe, etc. have masculine forms?
You can have une noire chatte or un noir chat, but you cannot have une noir chatte or un noire chat. The adjectives themselves have no gender, but they adjust to match that of the nouns, which are gendered. Just learn the forms the words can take and use whichever form matches the gender of the noun you are referring to. The reason you have chat vs. chatte is that a cat can be male or female. A tomato is just a tomato, meat is just meat and soup is just soup, so whichever gender is already assigned to it is the one it stays as.
Nouns that name things that can be male and female will usually have both masculine and feminine form.
Adjectives that describe nouns will have masculine and feminine form, to match the noun that they describe.
I'm not an expert in french, but this masculine/feminine aspect is similar to what we have in portuguese, so I guess this might help:
As a general rule, adjectives and articles have both masculine and feminine forms. Think of masc/fem in these cases as being similar to singular/plural, which means that they must follow the noun they are referring to.
For the nouns it's a bit more complicated. All nouns have a "gender". Feminine nouns usually end in "-e" (conversely, if a noun ends in "e" it's probably feminine).
Nouns that refer to things that can be male or female, like animals, usually have both forms.
For example: an apple isn't male or female, so there's just "la pomme", which is feminine and ends in e. A dog can be male or female, so there's "le chien" and "la chienne". Notice how the articles follow the gender of the noun.
If you add an adjective to the examples above, it must also follow the gender of the noun: "le chien noir" "la chienne noire"
Of course, like in any language, the rules come with tons of exceptions, and it's a matter of experience to get used to them.