An omen is an unusual event that foretells a greater event. Omens can be good or bad.
The translation of an omen as a good sign or a bad sign is often determined by the culture and background of the individual. If there is some question, various methods of divination may be used to clarify things.
Historically, hepatoscopy, haruspicy, lecanomancy, or libanomancy were most often used, though any method of divination may be employed.