What Case Does Sein Take

I read on multiple websites and according to what I have learned, sein is always followed by nominative. Examples: Er ist der Mann. Er ist ein großer Mann.

Is Sein always Nominativ?

You could say that sein is always accompanied by nominative; but seriously, there is (almost) always a nominative because the subject, which is obligatory, is in nominative.

How do you know if its nominative or accusative?

Nominative case is the case used for a noun or pronoun which is the subject of a verb. Accusative case is the case used for a noun or pronoun which is the object of a sentence.