How does a firstborn know to wash the kohen's hands?
Mishna B'rura 128:22 says that if no Levi is around to wash a kohen's hands for the latter's blessing, a firstborn-to-his-mother should do so. Although Kaf Hachayim :40 notes that he hasn't seen this done, I'm guessing some communities must have this custom. My question is how this works in practice. At musaf, no big deal: it's easy to see during the Torah reading (which precedes musaf) that there's no Levi. But in places that have the kohen's blessing at shacharis also, how does a firstborn know to go wash the kohen's hands?