Awesome project, congrats!
I had the same question before and ended up using the following approach with a CDN test tool to confirm that it was routed to the nearest region.
# config.exs
config :my_app, region: System.get_env("FLY_REGION", "unknown")
# my_plug.ex
conn |> put_resp_header("x-fly-region", Application.get_env(:region))
Anycast is amazing.
