Honestly I don't know how any Yacht, Boardwalk, or Beach Club guests could even know if you're staying at that resort. There's no IDing, or scanning Magic Bands - you have no way of knowing how many people at your given bus stop are staying at that resort or somewhere else. From Dolphin, the walk to Yacht Club's bus or Boardwalk's bus is probably close to the same distance, so you could pick either one. Depending on time of day and park, they sometimes have separate busses for Boardwalk, and sometimes combined with Beach and Yacht.
Boardwalk is my home resort - and I know between the Boardwalk entertainment and restaurants, and people using the Epcot area busses to get to Epcot via the Intl Gateway, that there are likely people at my bus stop not staying at Boardwalk - but short of asking them and getting an honest response, there's not really any way to tell and it's never bothered me. And I very often use other resort busses to get around - last week staying at Boardwalk, I walked over to DHS to catch the Wilderness Lodge bus, where I then took the boat to MK...coming back from Steakhouse 71 dinner, I took the bus to DHS which had just closed, and walked back to Boardwalk.
Long ago when Swan & Dolphin used to head direct to the Disney parking areas, the Epcot resort busses were often split with Swan & Dolphin getting their own, and Beach, Yacht, & Boardwalk getting their own. I would walk out my garden view room at Boardwalk and cross the bridge to Swan to hop their bus to MK, rather than take my own - not only was that bus stop closer to my room, but the bus made fewer stops and was more frequent! I stopped doing that when they started using the T&T bus stop instead of the MK one.