Is it possible to identify which user added, or subsequently edited, a particular feature?
Assuming you are using www.openstreetmap.org, select the object using either “Map Data!” in the layers menu, or the ? button. That will immediately tell you the last person to edit the object. Then click on View History at the bottom right of the data panel, to see the complete history.
In some cases this produces to much output, in which case you can use JOSM to request the full history, or you can access the history API directly, and get the information as OSM XML.
E.g. http://www.openstreetmap.org/node/2805796974 shows that you last edited the node in question and http://www.openstreetmap.org/node/2805796974/history shows that you were the only person to modify it at any time.
If you are lucky, there will also be a changeset comment explaining why it was changed and what information was used, but changeset comments are badly under-used.
For ways, this does rely on people trying to modify existing ways, rather than adding new ones, when extending.
Thanks for your help.