Someone can be declared as Taghut (false god) in case that they call to Shirk (associating others with Allah in His Divinity or worship), to offer `Ibadah (worship) to them, claim knowing matters of Ghayb (Unseen), or judge according to anything other than what Allah has revealed and the like.
Ibn Al-Qayyim (may Allah be merciful to him) said:
Taghut is anything in which a servant exceeds the limits whether in the field of `Ibadah, following, or obedience. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.