The title says it all - how can you tell what folder a report is in using SOQL?
As of API 35, the Report object has FolderName field.
SELECT id, name, ownerId, folderName FROM Report
OwnerId is the ID field from the Folder object, which is the Report Folder the report lives in.
FolderName field is the label of the folder and not the unique name. So if you wanted to query for reports by their folder's unique developer name then filter by
OwnerId instead like in Daniel Hoechst's answer.
SELECT id, name FROM Report WHERE ownerId IN (
SELECT ID FROM Folder WHERE developerName = 'FolderName'