A way to measure a signal to determine precise FM deviation is with the Bessel Zero method. It uses carrier null to indicate deviation. The following describes the process I used to calibrate the PRO-2026 scanner and deviation interface box combination shown elsewhere on this web site.
An FM signal generator with a variable frequency audio source along with a communications receiver is required. For best results the communications receiver should have an analogue "S" meter and a narrow (CW) filter. We are looking for a carrier null that occurs with certain modulating frequencies and at a certain deviation. As the modulating frequencies get lower, the sidebands get closer to the carrier frequency and it becomes difficult to determine the carrier null when the sidebands fall within the receiver bandpass. This is the reason for the narrow filter. The "S" meter reading is used to indicate carrier null. Set the receiver AGC to fast for a quicker "S" meter response. Set the communications receiver to the "AM" position. A high quality signal generator should not be a requirement as we are not looking for accurate signal levels.
The signal generator frequency is not critical but it will need to fall on a frequency that both the deviation monitor and the communication receiver covers. Determining a carrier null on the communication receiver and then changing the signal generator frequency to fall on a different deviation monitor frequency may not produce the same deviation.
Connect the signal generator to the communications receiver and adjust the RF output of the signal generator with no modulation to a suitable level on the "S" meter, S9 for example. Tuning the receiverís IF shift or bandpass tuning may help to peak the signal when a narrow filter is used. Pick a modulating frequency. A thousand Hertz is a good choice to start. This will produce the first carrier null at 2405 Hz deviation. I made a spreadsheet done in XL format that can be used to determine the relationship between modulating frequencies and carrier nulls. The spreadsheet should be correct but I do not guarantee the accuracy. You can save it by clicking here. Starting with no deviation slowly increase the level of the modulating frequency (1000 Hz in our example) until the "S" meter reading starts to drop. There will be a point where a null will be produced and the "S" meter reading will start to increase again. Where the null occurs the deviation will be +/- 2.405 KHz. It will be possible to get more than one carrier null. The first carrier null encountered is the one we want in this example. Remove the RF cable from the communications receiver and connect it to the deviation monitor (scanner) input. An adjustment can now be made on the deviation monitorís calibration control(s) to indicate +/- 2.4 KHz deviation on the oscilloscope. Other modulating frequencies can be tried to determine the monitorís overall accuracy.
UPDATE: November 19, 2015
I tried substituting the receiver with a NooElec dongle and the SDR-Sharp software. It works very well.