You have a very good point there and I have opened a feature request internally to display the ID somewhere.
There are 2 options I can think of, You could open the workflow you want to use and add a set node then set it to use an expression and for the value input {{$workflow.id}} this will show you the ID or while the desktop app is open you can open your browser and go to http://localhost:5679 and this will give you the n8n interface in your browser and you will be able to find the ID in the URL.