@eidling  One clue would be if your WiFi signal is suddenly stronger in places distant from your main router than in the past.

The more definitive way to tell would be actually signing into the signal from the extender. You would not see it if you gave it the same SSID as your router's WiFi. If this is the case, change the name, and make sure you can both see it and connect to it. (In my case, I can see it, but I cannot connect to the 2.4 G Band). 

(Note I am not with TP-Link. I'm just a new purchaser who feels sympathy for others with problems after spending hours trying to get mine to work yesterday, and am checking in here in case there was a holiday answer to the question I left.)