The custom canvas created by a user gets stored against the customCanvasState key that you pass to Tokens API. This is explained in detail here. In order to see the dashboard that a user has created, simply generate a token using their embeddableId and their customCanvasState and use that to render an embeddable and you’ll see exactly what they see. You can even pass customCanvasReadOnly: true to the Tokens API to make sure that you’re only seeing a read-only view of what they see.