I've done things like this before, and it usually works.
Ring the bell. When they answer the door, tell them you were riding your bike and saw a round thing in their yard. Show them the dateless nickel and say, "I thought you might like to have this." Then give it to them (Buffalo nickels that worn aren't worth much anyway). See what kind of reaction you get.
If the reaction is favorable, tell them you metal detect for a hobby and if they would mind if you checked their yard with it.
If they say that would be fine, you have a yard.
If they say no, tell them "Thanks anyway, I didn't think it would hurt to just ask." Then leave.
If they have refused you AND they end up taking your nickel, send me an email of your experience and I'll send you another WITH a date on it.
Nat