How to identify dynamic ips in ipv6?

Are you referring to a local instance or are you trying to verify if the people who access your website / content do so from dynamic ips? For dynamic ips, are you really wondering if they are DHCP, or something else?

Maybe a little more information about what would be the desired objective, since you will have difficulties to try to find out if they are robots or false clients without further information to continue.

But to help a little with what is provided, you can use cookies and see if there is a cookie for a client, but with a new ip. If that is the case, and it is in a similar network, then you would know that the client is probably assigning a new IP address from the same computer.