"Il y a des drapeaux aux fenêtres."
Translation:There are flags in the windows.
Fenêtre is a female noun so if it was in the singular it would be à la fenêtre. If it was a male noun starting with a consonant it would be truly impossible to distinguish between singular and plural from the phonetic type only. Female nouns, however, use à la in the singular while female and male nouns starting with a vowel sound use à l'.