Most of the time, by using a drive test tool (google TEMS Investigation) connected to several MS (one MS per operator).
The tool will meaure the QOS for each operator at the same time, allowing to perform comparison.
This can be done for one operator only, in this case it’s just monitoring/testing the qos of one network.
But the test can occur at different level, at different interfaces, not only Air interface. That would be done with protocol analyzer such as tektronix K12.