θεὸς, like λόγος, is a member of the —ος family of masculine nouns. All nouns may appear either with or without the definite article. If the noun HAS the definite article, it is always easier to identify its case and gender. But the NT writers often do not use the article, so we must do our best to recognize nouns just by their word endings alone.
And while θεὸς is most often singular in the NT, we definitely find it appearing in the OT in a plural form, as in Jeremiah 16:20 Can people make their own gods? No, what they make are not gods at all.
So we must be prepared to recognize all members of the —ος family, including θεὸς, by their suffix alone.
Following are questions about several Greek words. If you don't know the answer go ahead and guess.
What sort of word is θεοῦ?
What sort of word is
θεόν?
What sort of word is θεῶν?
What sort of word is θεοί?
What sort of word is θεῷ?
What sort of word is θεούς?
What sort of word is θεός?
What sort of word is θεοῖς?