Is there a way for any of them to actually prove it technically?
No, it's not possible for a VPN provider to prove they aren't keeping logs, and it's pointless to try. It doesn't matter how well-intentioned the operators are either - they can always be tricked or coerced into keeping the logs and forbidden from telling you about them.
If you're using a VPN for privacy reasons always assume they are logging all your traffic and adjust your usage accordingly.
Pick one that lets you use bitcoins to pay, and allows you to connect via Tor.