There are a lot of applications and web pages offering to test the quality of your internet connection, mostly by performing tests that will show you your download and upload speed, as well as the latency of the line.
Some examples are:
Speedtest by Ookla (this is probably the most famous one)
Internet Speed Test by Netflix
Meteor by OpenSignal (only iOS and Android)
OpenSpeedTest
SpeedOf.Me
nPerf
If you run tests on several of these tools, you may observe significant deviations in the results. Such differences emerge because of several factors: the measurement server location for each tool (near or far away from you), the suitability of the server (e.g. having enough available capacity to serve multiple users at the same time), the exact type of tests performed, brief network fluctuations, or even traffic spikes due to other people watching YouTube or Netflix on your line.
Multiple measurements with time spacing between them should give you a better idea of your line’s performance and flatten temporary network glitches.