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I have seen this a few times. Really quite simple. Just like the Gabbai will, unless he knows everyone, ask "Is there a Cohen?" or "Is there a Levi?" when it comes to Layning, he will do the same, ...
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#1: Initial revision
I have seen this a few times. Really quite simple. Just like the Gabbai will, unless he knows *everyone*, ask "Is there a Cohen?" or "Is there a Levi?" when it comes to Layning, he will do the same, if needed, for Duchening. Either "Is there a Cohen?" to find out if there will be any Duchening, or "Is there a Levi?" to wash the Cohen/Cohanim hands. And if the answer is "no" for "Is there a Levi?" then, unless he is himself a Bechor, ask "Is there a Bechor?" I know at least one person who is a Bechor (and not a Levi) who jumped at the chance to do this Mitzvah when he had the rare opportunity to do so. (And for those who are wondering, I am a sometimes Gabbai, but despite being a Katz, I am not a Cohen.)