How can I see which orders have already been assigned to a route?

On your EasyRoutes for Shopify Orders page, there is a column at the far-right of the table titled View Route. Any orders that have already been assigned to a route will have an icon in this column which you can click to view the route they are contained in:

You can also use the Route filter to show only stops that have a route (or, alternatively, are not yet routed):

On EasyRoutes for Web, using the Route filter works the same way; however, the View Route button is nested within the Actions menu (three dots icon) for each imported order's entry: