The following is a list of IPv6 testing sites that I find useful.
And then test whether your IPv6 enabled servers are accessible from the wider IPv6 internet.
IPv6 Status Check (Web, E-Mail, DNS and NTP)
IPv6 Health Check (All DNS servers, Web, E-Mail and SPF record)
IPv6 only email bouncer – send an email to email@example.com and if you receive a reply then you have successfully sent and received email via IPv6.
Other tools can be found at Freenet6 IPv6 Tools.