You simply ask the good sameritan if they own a boat and what size. If they pass your minimum size perhaps they can handle the bitter end... if not it's, not worth the risks.Over the years I've found that help from strangers on a dock can be more of a problem than help.
Bowlines tied to a mid-dock cleat while entering a slip, knots that don't hold, lines dropped into the water, etc. We do it ourselves, and just say, "Thanks, but we've got it".
Many boats can be "docked" along side with no help ie single handed. Use a mid ship line and tie off to a pile or cleat and that secures the boat. With the bow and stern lines set up over the mid ship position... once on the dock you can grab them one at a time and set the bow and stern. This is probably OK for a quickie stop... longer add spring lines.
Color coded lines help...