Something like BitPay and Coinbase coming out and detailing their merchant trade volume in each of the currencies they facilitate would be the best possible evidence.

Dogecoin used to do loads of transactions via the Reddit tip bot. The first one is now defunct (was running a fractional reserve and lost everyone's money), the second one does barely any transactions these days:

https://www.reddit.com/user/sodogetip

There are other doge tip bots on Twitter and other platforms but they're not doing loads of transactions as far as I'm aware today.

Dogecoin actually used to have pretty good merchant adoption compared to most cryptos, but that was several years ago. If you go through this page now they are largely defunct or inactive:

https://www.dogecoins.com/spend.php

In the absence of evidence of any significant activity, I think it's fair to assume the Dogecoin transactions are mostly self-transfer spam.