A scanner next to the generator
Most QR code maker sites skip this page. That is a mistake. The only test that matters is another device reading the file you are about to send to a printer.
Point the camera at a screen, a proof print, or a photo. If the payload is a URL, you can open it from the result panel. If it is Wi-Fi or a vCard, you will see the raw text - enough to confirm you encoded the right SSID or phone number.
This is not a replacement for the camera app in your pocket. It is a proofing tool for people who just made a code on the desktop.