Over on YouTube user KP4MD has uploaded a video showing how she uses an RTL-SDR together with SDR#, a program called Visual Analyzer and an AEA PK-232 Terminal Node Controller to measure the frequency deviation of a Yaesu FT-8800R Transceiver. She writes:
The SDR# receiver is tuned to 145.050 MHz and the bandwidth set to 20 kHz.
The deviation level of the 1200 Hz tone is increased until a null appeared on the carrier frequency.
This is called a Bessel Zero and occurs at various predicted modulation indices (2.4, 5.52, 8.66, etc).
The Modulation Index is defined as the peak frequency deviation divided by the modulation frequency.
This Bessel Zero occurred at a modulation index of 2.4 corresponding to a frequency deviation of ±2.88 kHz (2.4 x 1.2 kHz).
The oscilloscope indicates that a peak to peak amplitude of 54.3% corresponds to ±2.88 kHz deviation.
The 1200 Hz tone modulation is increased to yield a peak to peak amplitude of 66%.
This corresponds to the desired ±3.5 kHz frequency deviation.