The following is a list of IPv6 testing sites that I find useful.

First to test your connectivity as a client (can you reach IPv6 websites, DNS, email servers, etc)
Test-IPv6
IPv6-Test

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 bouncer@freenet6.net 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.