Every new website/product/service claims they respect privacy and don't store your information on their servers - how can someone without profound technical abilities verify this? It can't just be blind trust, can it?
With physical products that you connect to your network (eg. IoT devices such as smart light and other things) you can usually check dns requests. If you have something like pi-hole or Adguard installed, dns requests will be directed those and you can check your logs. You should of course block any other dns request in your firewall so hardcoded dns won’t work.<p>If you use a website it is hard (or impossible) to know if your data is secure and private.
One way to spy on an app's traffic is to setup NextDNS with logging turned on, and then watch it as you open the app, see what it talks to. You can't see the actual content, only DNS requests, but DNS alone can sometimes be enough to 'catch' the app phoning home.