Curses are labeled as "curse" in the descriptive text and has text describing exactly how it interacts with the Remove Curse spell.

Monster Example:

Monster Manual page 229: A creature that takes treasure from the lair is cursed until the treasure is returned. The cursed target has disadvantage on all saving throws. The curse lasts until removed by a remove curse spell or other magic.

Item Example:

Dungeon Masters Guide page 152: Curse. This armor is cursed, a fact that is revealed only when an identify spell is cast on the armor or you attune to it. Attuning to the armor curses you until you are targeted by the remove curse spell or similar magic: removing the armor fails to end the curse. While cursed you have vulnerability to two of the three damage type associated with the armor (not the one to which it grants resistance).

Curses are labeled as "curse" in the descriptive text and have text describing exactly how it interacts with the Remove Curse spell.

Monster Example:

Monster Manual page 229: A creature that takes treasure from the lair is cursed until the treasure is returned. The cursed target has disadvantage on all saving throws. The curse lasts until removed by a remove curse spell or other magic.

Item Example:

Dungeon Masters Guide page 152: Curse. This armor is cursed, a fact that is revealed only when an identify spell is cast on the armor or you attune to it. Attuning to the armor curses you until you are targeted by the remove curse spell or similar magic: removing the armor fails to end the curse. While cursed you have vulnerability to two of the three damage type associated with the armor (not the one to which it grants resistance).