As another commenter said, it’s not possible to verify. You’ll just have to take each instance’s word for it.
Instance lists for some privacy front-ends will point out additonal info, such as if each instance is using CloudFlare or not (this may or may not be useful depending on if you distrust CloudFlare), some other services (like Rimgo, a private Imgur frontend) lets the instance hoster customize the privacy policy. But once again, this is all relying on the instance host telling the truth.
Use Mullvad, unless you absolutely require port forwarding.