There are speed tests (either from third parties or from operators) to measure the ISPs’ actual speeds. Examples are: 

• https://www2.telenet.be/residential/nl/landingspaginas/speedtest
• https://www.proximus.be/en/id_cr_speedtest/personal/orphans/test-and-optimize-your-internet-speed.html
• https://www.orange.be/nl/opties-en-diensten/speedtest
• https://www.speedtest.net/
• https://www.test-aankoop.be/acties/internetspeed/test

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:  

1. always use the same speed test (this avoids deviations caused by other speed tests); 
2. establish a fixed connection between the measuring device (laptop) and the modem (this avoids a speed that is too low due to the use of Wi-Fi restricted by walls or other electrical equipment);
3. make sure that other persons in the household do not use the Internet at the time of measurement (they also take up capacity that cannot then be used for the measurement and thus reduces the result of the measurement);
4. close all other programs (browser, gaming, music) on the measuring device (laptop) (even those programs use capacity which causes the measurement result to be too low);
5. measure your speed between 7 p.m. and 9 p.m. several times and for several days (to prove continuous or frequent inadequate speed);
6. take screenshots of all measurements and save them along with the date and time of each measurement.