It would depend on the accuracy and resolution. The best way is a benchtop frequency counter with at least a 10 second sampling time and preferably a 100 second sample time. Note that the second choice is very expensive.
The 10n second sampling time will offer a resolution of 0.1 second so that you might get 3.6 Hz, 3.9 Hz, 4.2 Hz, etc.
The frequency of most Beck units are not particularly accurate.