Tus-js-client sends a HEAD request (see Resumable upload protocol 1.0.x | tus.io) to check the upload’s status. On the server-side, there are no events so far to modify the server’s response.
That’s currently not possible with tus-node-server. @Merlijn and I have been brainstorming about how this situation can be improved using metadata. The server could attach metadata once the upload is finished, which the client can then retrieve when either the upload is finished or the client notices that the upload is already done.