If you consider an omen to be a kind of sign, then you can say:
Gangsters use hand-signs to challenge other. This is known as “hitting up" or “waving hi."
|( に or を ) いど＊む||
to challenge someone or something to a win-or-lose battle. Sports or sometimes politics. （challenge the world champ a fight / challenge the system)
challenge (challenge yourself to do better, test your limits / challenge the system) be defiant or provocative. take the pepsi challenge!