Today i found great resource for the IPv6 adopters. NAT64 Check website checks and compares website look and feel in NAT64, IPv4-only and IPv6-only modes. This way i been able to find that on my of the my web sites IPv6 was not configured correctly (browser was automatically falling back to v4, so i did not found that myself). Another interesting issue is some external scripts hosted on public CDN-s. Some of them do not have IPv6, e.g. very popular maxcdn.bootstrapcdn.com.
Service works by downloading all resources in 3 different network environments. Also it is trying to compare rendered image of the web site to find, for example, broken geo based lookup or other issues, like incorrect virtual host configuration which will cause different rendering result.
Thats how it looks if everything is fine:
So, thank you Jan Žorž, and lets make more websites IPv6 ready 🙂