I most often check speedof.me but they're all different to some degree even when run simultaneously.
If you ask your ISP, they'll tell you the speed you're buying is the very best you can ever possibly get, and "overheads" will reduce it to a much lower figure. We consistently get about 60%-70% (wirelessly) of the rate we pay for and we've got the latest modem and router our ISP recommends. Well, of the recommended ones ... there are several.
My 2016 MBP is connected via Ethernet and it gets about 90%+ of the advertised speed.
Ookla and Comcast are others. There are many more.