Basically just traceroute, and perhaps combined with looking up your isp's peering information (and their peer's peering information, depending on how deep you

Basically just traceroute, and perhaps combined with looking up your isp's peering information (and their peer's peering information, depending on how deep you want to investigate them) on peeringdb.com . For example, you might discover your isp will use route traffics to a certain website via a slow peer based on your traceroute information and data on peeringdb.com. But like you said, it's not useful to us as a customer of an ISP because we can't alter the routing table used by the ISP even if we identified a problematic routes/peers. You can try to complain to their customer support, and might get lucky if the customer support has technical authority to debug peering issues.