Bonjour Raman deep,
That's an excellent question! Most of the time, the context will be helpful and will give you a clue. It's also important to know you should use a "liaison", i.e. when two words are linked together in speech.
Je vais aux États-Unis. = I am going to the USA.
Je vois des oiseaux sur une branche. = I can see birds on a branch.
Voici une sélection de thés verts aromatisés. = Here's a selection of flavoured green teas.
-> you could do the liaison in the last example but it sounds a bit formal in French
Attention: de and des are pronounced differently. Listen to the examples from the following lesson: Use de or d' instead of des
Also, take a look at one of our partners' site on liaisons: https://www.lawlessfrench.com/pronunciation/liaisons/
I hope this is helpful.
Bonne journée !
