Following commands are for a machine running Linux (more precisely: Linux Mint, derived from Ubuntu).
Getting IPv6 address
Use the ifconfig command. Take the inet6 address with global scope.
To check that this address can be reached from the Internet: https://www.ipaddressguide.com/ping6.
Checking outgoing connectivity
# Ping Google DNS.
ping6 -c 4 2001:4860:4860::8888