There's not really a consistent way, that's why I said "ïf". If they have a single gold line either around the sleeve of their dress or jacket, then they are lead-qualified. If there's only one of those up front, then that's your lead. If there are two or none, then you have to guess. It's usually the more senior of the two (senior people generally like to fly lead or in the back) so if they have a five or ten year service button then it's a fair assumption. When I'm boarding and asking a similar question I'll often hedge my bets by making eye contact with both as I speak.