Many options are possible to tune your AF level. obviously measurement tools like ocsilloscope tap directly at the output of the Af Amplifier or a spectrum analyzer is also very useful.
in another hand you can use also another RX, a SDR receiver or even a Websdr online in order to monitor quit accurately your transmission. When you find the correct level without distorsion spikes you just need to calibrate a vumeter with the suitable value and threshold. The UBITX does not have a AF level input monitoring
and without an additional device to monitoryour level it is simply impossible to know if the level applied from the Microphone preamp is not too high or too low.
in the picture below you will notice a little switch just below the vu-meter in order to monitor the AGC and AF input level.
73s F5NPV / Didier