As a language learner, you have to be on the lookout for nouns that don't change in the plural. This mostly concerns masculine and neuter nouns ending in -el, -en, -er. Some examples:
der Nebel, Spiegel, Zweifel – das Kabel, Mittel, Viertel
der Brunnen, Knochen, Streifen – das Eisen, Wesen, Zeichen
der Bürger, Koffer, Treffer – das Fenster, Lager, Zimmer
For count nouns, the presence or absence of a determiner will help distinguish singular and plural uses. ("Nullartikel" written as ∅.)
Das Programm findet ∅ Rechtschreibfehler. (plural)
Das Programm hat einen/den Rechtschreibfehler gefunden. (singular)
Unfortunately, in newspaper ads, articles are optional even for count nouns. But singular and plural can still be distinguished by the adjective ending. The rule you gave for the ending of the adjective is correct.
Suche ∅ antikes Möbel. (singular, -es as in das, welches, etc.)
Suche ∅ antike Möbel. (plural, -e as in die, keine, etc.)
Finally, note that many speakers prefer to use Möbel in the plural only; Möbelstück is used for a piece of furniture.