How can I make sure this is working when I don’t know anybody who has installed it yet?

There is a special ID named ECHOECHO that simply echoes back all messages that you send to it. Scan this QR code to verify its key:

Note that in general, QR codes for ID verification should not be sent over the Internet but only scanned from the personal mobile phone of the respective person in order to ensure that there has been no manipulation by third parties.