If you turn out to be wrong, even if the mate is forced but in a different number of moves than you announced, you'll look somewhat foolish. So why even think of doing it? Let the moves speak for themselves.
In a tournament, it is definitely not allowed, because you are not allowed to speak at all, except to offer a draw or resign. (You don't even say, "check."
So announcing a mate in a tournament is against the rules.