How may I find all of the files modified outside of Visual Studio 2017 which have changes TFS cannot "see" as they didn't happen in Visual Studio 2017?

In Visual Studio 2017, go to the Source Control Explorer (from the Source Control Explorer button at the Team Explorer pane (found with Team Explorer under the View menu)) and right-click on the appropriate folder which is the umbrella over everything and pick "Compare.." ...also here you will need to right-click on each item that appears and do the "Check Out for Edit..." trick described here to make the files something that can be seen by TFS for check in.