It's really just like a regular query:
If you know the position of the newly dropped annotation:
// say your annotation answers a location with lat and long
PFGeoPoint *point = [PFGeoPoint geoPointWithLatitude:lat longitude:long];
Query your custom class for nearby instances like this:
PFQuery *query = [PFQuery queryWithClassName:@"Pins"];
[query whereKey:@"location" nearGeoPoint:point];
// do your regular find stuff with query.
In case the question is about js (it's unclear from the tags), its the same concept, using:
var Pins = Parse.Object.extend("Pins");
var query = new Parse.Query(Pins);
query.near("location", point);
Or to do it around a given radius, then:
query.withinKilometers("location", point, 0.1); // 100 meters
query.find().then( /* return to client */ )