There are speed tests (either from third parties or from operators) to measure the ISPs’ actual speeds. Examples are:
You will find several others on the Internet.
Once you have selected one, it is in your interest to obtain as accurate as possible evidence of the inadequate speed.
The following conditions have a positive effect on the accuracy of the measurements:
The BIPT emphasises that even with all these tips in mind, there are still the limitations of the measuring device itself. For example, to measure a speed of 1 Gbps, the laptop must have a network card capable of achieving a speed of 1 Gbps. With more modern and/or expensive laptops, this is often possible already, but with older and/or cheaper models this may not be the case.