On the modem side, nothing needed to be done for IPv6. Any firmware updates or configuration needed is done by Comcast.
On the router side, there is a dedicated IPv6 configuration page. Settings are:
• Connection Type: Native
• All other available settings: Set to either 'Enable' or 'Stateless'.
That was it. My Mac has IPv6 configuration set to configure 'Automatically', and I get an IP. If I go to plain Google and ask it "what's my ip" I get a IPv6 IP back.
On my iPhone, on wifi, if I do the same "what's my ip" Google search, I get an IPv6 back. My carrier is Ting GSM; if I turn off wifi and do the same Google search, I get a different IPv6 IP back.
At work, IPv6 is rolling out _very_ slowly, but I was able to get a fixed IPv6 address. That has been programmed into the configuration for my laptop's USB-Ethernet adapter, so I have IPv6 at my desk at work. Although most work services are not IPv6-enabled, DNS is.
And at home, IPv6 is enabled and is in active use, both on desktop and on mobile.
As for testing, I suggest https://test-ipv6.com and https://ipv6test.google.com