Mist UI has options to run speed tests on wan ports under wan Edge interface. These speed tests can be triggered on demand or can be scheduled to be run.

The speed test feature has limitation that it can only provide accurate results for links upto 1G speeds. Mist will however let the users run the speed tests on links over 1G speeds but the results may not be acurate

https://www.juniper.net/documentation/us/en/software/mist/mist-wan/topics/task/wan-edge-speed-test.html