Ask HN: How to bandwidth test a gigabit connection?

I've purchased a gigabit broadband connection at home, marketed as '1Gbit/s'. It is very fast, but I am unable to determine how fast. Is there a way to independently verify a connection like this? Preferably with a neutral third-party.

Mainstream testing services like Speedtest.net give wildly different results, which are rarely more than a couple hundred Mbit/s (many Speedtest.net servers have less than a 1Gbit/s available). I considered iperf but my servers are capped at 100Mbit/s so this is pointless.

Anyone else with gigabit connections out there able to fact-check your ISP's marketing department? If yes, how do you do it? I don't need to see perfect results but I expect to see better than 800Mbit/s consistently.