As Sanchez says the first way to know it is to :

• activate developer mode (more details)
• go to the view you want to know its id 
• click on debug menu and choose "Manage Views" option 

That will display the list of all model views, its sequences, types and external ids (what you are looking for). Displaying the list of all views and its type is useful to know if there is an other module that inherits this view to set the correct priority of yours. 

The second way to achieve the same list is to use settings menu -> Technical -> user interface -> Views and filter by model or another criteria.

The last one is to go to the source file of the module and check in xml view source file, but you can't know by this way if there is another view from another module that inherits the view you are looking for.

Hope it helps!