If one is uncertain about the contents of one’s dream and is unsure if it is a harbinger of good tiding or bad ones, one should employ the standard supplication during the blessing of the Cohanim (as now printed in almost all prayer books).
In most communities it has become customary for the entire congregation to recite this supplication, which or not each individual has recently experienced a dream. Everyone has seen an ambiguous dream at some point since the last time the Cohanim blessed the congregation.
It has become customary for the Cohanim to recite an extended, wordless chant at the last three words of the blessing. Rama (O.C. 128) writes that it is during this chant that one ought to recite the dream supplication. One does not recite it while the Cohanim are actually reciting the words of the blessing being pronounced on their behalf.
Torah Insights Revealed During a Dream
Many great Torah scholars had Torah insights revealed to them in their dreams. Generally one may only decide halachic questions based on existing Torah sources, not on divine revelation; nevertheless, if the ruling received in a dream does not contradict those sources, some consider it valid. In fact, there was a Tosafist by the name of Rabbi Yaakov of Marosh who asked many questions via shaylas chalom, i.e., a question posed to HaShem for which one hopes to receive a response during a dream. Rabbi Yaakov received 89 responses via dreams; he recorded them and they are published under the title: Shailos U'teshuvos min Hashamayim ("Responsa from Heaven").
