I always thought that giving a siman for something in advance is ok (i.e., Eliezer’s siman regarding if a girl would offer him water and offer to water his camels as well, that it would mean that she was the chosen bride for Yitzchak Avinu. However, it is assur to say after the fact of an occurrence,i.e., a bird suddenly flies overhead and lands on your shoulder, “Oh, that is a sign that such and such will happen.”
