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Make sure your equipment supports IPv6 and is properly configured to handle IPv6 traffic since if not then it won't matter if the ISP has IPv6.
Fully native IPv6 is always preferred over transitional IPv6 connectivity although its important to determine if the ISP is just using tunnels for IPv6.
Use Hurricane Electric's World Report (seems to be auto-generated) and pull a report for the country you're in. Then compare ISPs that are available to you. The 'Adjacencies v6' column refers to how many other IPv6 connections to other companies they have (peering) which will improve performance. The 'Routes v6' column refers to what portion of their network is available over IPv6. Both columns you can compare to the related v4 columns to get an idea what percent is IPv6 now as well. Now it should be assumed but if the ISP has no v6 routes listed then they don't offer IPv6 to their customers currently.
If the ISP has a very low ratio of v6 to v4 routes then they may have a limited IPv6 deployment say only in trial areas or via Relay Services.
If you are a current customer of an ISP without native IPv6 connectivity then make sure to tell them of your interest in that. For business service, then contact who handles your account. For residential service, the message could easily get overlooked if handled by customer-facing agents so make sure they send it up to someone with some decision making influence.
If your current ISP doesn't support native IPv6 then you could consider using a managed IPv6 over IPv4 tunnel through a tunnel broker. All of those will reduce performance especially if it has to overcome NAT traversal.