The first indication you have data processed meeting Collections specifications is that your data will have the new Landsat Product Identifier, which replaces the Landsat Scene ID.
Along with information inherited from the Scene ID, the new Landsat Product Identifier also includes the processing level, processing date, collection number, and collection category (noted in red in the graphic below.)
The Scene ID will remain in the metadata file (MTL.txt) delivered with the Level-1 data product. The heritage Scene ID will also be visible and searchable (along with the new Collection 1 Landsat Product ID) in the online metadata on EarthExplorer.
The Landsat Collections Web page contains the details about all aspects of the Collection 1 effort.