As the previous poster said, use speedtest.net. Make sure that if you're running Safari you have the latest version of Flash installed. Flash Player is switched off by default by Apple, so that might be why you're having problems. Once installed, you need to switch it on in your Safari preferences.
"Cable broadband is typically faster than copper-based services, but a significant minority of users experience severe slowdowns in peak times." — Ofcom, UK Home Broadband Performance, November 2016