Didier F5NPV notes that many options are possible to tune your AF level. Measurement tools like an ocsilloscope can tap directly at the output of the AF Amplifier. Alternatively a spectrum analyzer can be very useful in monitoring spectrum quality.
It is also possible to use another RX, a SDR receiver or even WebSDR online to monitor your transmission in quite an accurate manner. When you establish the correct level of gain required to generate a signal without distortion spikes you just need to calibrate a meter with the appropriate threshold value.
The µBITx does not have an AF level input monitoring system. It is simply impossible to know if the level applied from the microphone preamp is too high or too low.
in the picture above you will notice a little switch just below the vu-meter thatI use to monitor the AGC and AF input level.