Your Duet WiFi will either have a blue LED on the WiFi module, or a green LED next to the WiFi module. This flashes once when power is connected, and is lit permanently when connected to a WiFi network. Duet 2 WiFi with external antenna has no WiFi LED. If you have a Duet with external antenna, don't forget to plug in the external antenna!

If you didn't turn off WiFi, you may find you are getting a lot of messages like "WiFi reported error: no known networks found". The default configuration turns on the WiFi, but it is not configured to connect to a network yet. To stop the WiFi, send M552 S-1

Send M552 to check the status. It should say "WiFi module is disabled". Now put it into idle mode by sending command M552 S0. Send M552 again to check it's in idle, then proceed.