Unless you have access to their equipment, you can't. All you can really do is document your throughput over a reasonable length of time, like a week, and collect statements from other users.
I don't think you necessarily have a case, however. Since during off-peak times your connection performs as it should, you can assume that there's no technical issue with the line itself and that your ISP is in fact providing you with the connection you signed up for.
Note that when you buy a particular speed connection from a service provider, that speed only pertains to the line between your location and the ISPs border router; your ISP has no control whatsoever what happens to packets once they're out on the internet.
Therefore, what you're describing does sound like latency to me. Your issues happen during peak hours, when most of your ISPs other customers are using their internet connections simultaneously.
However, depending on the type of "downloading" you're doing, your traffic might be getting lower priority causing your throughput to appear to be slowing down. In that case, what you're calling "throttling" is actually called packet shaping.
ISPs have every right to limit, for instance, P2P traffic on their networks, because otherwise it can literally cripple a network. There's no way an ISP can allow all its users to have full bandwidth on any protocol they desire, because then a very few would make decisions that would result in not just latency but downright outages affecting hundreds or thousands, and no service provider can afford to tolerate that.
If you want a circuit that gives you unrestricted access to all protocol types at all times, you will need to pony up some real money and lease a private line directly from a CLEC. It could cost hundreds or even thousands per month, but there'd be no packet shaping and you wouldn't have to share.
