Mainly, we distinguish the case by the article.
Nominativ, the case of the subject, is identified by
der/ein, die/eine, das/ein,
and for the plurals, by
die/--
(as in English, there is no plural form for the
indefinite article: a dog/ dogs).
There are some regularities in the formation of the plurals,
but basically you have to learn them by heart, together with the article.
der Geist, die Geister
der Tisch, die Tische
die Hand , die Hände