As someone who is in DC doing net neutrality advocacy on a daily basis, I love this question. One of our major challenges is that ISPs have data on their networks and we don't. That leaves us in a position to speculate about what is "really" happening and hoping that someone out there happens to have the technical skills to potentially uncover a problem. The HN community's ability to evaluate claims made by ISPs as legitimate or illegitimate based on technical constraints is incredibly valuable.
Interestingly though, there is an argument that even ISPs don't have the right data, and that this is a reason - or at least an excuse - for throttling. See http://www.bobbriscoe.net/presents/1005ftw/1005ftw-briscoe.p...
There is an ietf working group (ConEx) on exposing the kind of information necessary, with the idea that this would provide the right kind of incentives for ISPs not to throttle. I'm not sure that ISPs wouldn't anyway have the incentive to be a gatekeeper, but it would be a good idea to remove any possible excuse.
Or, you could refer to the product you're interested in buying, as they can provide information about how well different internet providers can service their product:
