how do you know to use "les" instead of "des"

Question

Accepted Answer

Hi Judith,
This understanding comes from studying the French grammar "articles"
The key is to be able to identify the category of the noun before which is placed an "article".
"les" is placed before plural nouns as a definite article. 
"des" is placed before plural nouns as an indefinite article. ie. Un garçon a boy , des garçons some boys. 
The subject is too extensive to explain in a post like this  --  you simply need to study "Articles" to enhance your overall understanding.
https://progress.lawlessfrench.com/revision/grammar/using-le-la-l-to-say-the-definite-articles
Hope this helps.
Jim

Answer

Bonjour Judith,
Here are another couples of links to supplement Jim's answer: 
plural-of-the-and-a-les-and-des-articles
du-de-la-de-l-are-used-to-express-some-or-any-partitive-articles
I hope this is helpful.
Bonne journée !