I’m sure anvil.server.context.client.location could help with this, since a pc or phone logged in on an anvil page connected to the same wifi as the pico should have the same location information as the pico.
…now that I think about it, you could probably just get away with an authenticate button on an anvil page, after giving the user instructions to connect to the same wifi as the pico and thats really the easiest answer to the OP question.