With the advent of V8 format files and the idea of more than one model in a DGN, the user does not know that there is more than one model in a file unless the Model dialog is opened.
It would be very helpful to provide a visual aid or notification to the user indicating that there is more than one model in the file or perhaps the number of models.
The Blue icon that is used to open the other models ( if there is not going to be model tabs) should change colour say red or green or even yellow if there are any models other than the default, and stay blue when only the default or single modelexists. But the Home ribbon may not be displayed when opening a model
Put the number of models in the title bar, perhaps in a different color
Place an icon in the Quick Access Toolbar
The models could be displayed in tabs either directly below the ribbon or at the bottom (like in MS Excel). See attached picture
20 years after the ability to have more than 1 drawing model was added it is still not recommended to use this feature (except in cell libraries) since other users won't automatically know that there is more content than in the default model.