If you want really high accuracy and you have a Varying frequency sensor than a oscilloscope would be the best way to measure the sensors output as it will display the frequency waveform.
If this is just a verification thing you can search MAF tap test to see if that type of test would be sufficient for your purposes.
Some multimeter have a frequency measurement mode, this could work for you but won't show quick fluctuations in the frequency waveform just a sort of average frequency.
Hope this helps.