The answer depends on how quickly / often you need to know the frequency?
If you can afford to measure lots of pulses, and wait a while, then you can simply calculate the time for say 1000 pulses, by counting say the rising edges, and using a stopwatch. Your edge count has a max error of 1 (either an edge has happened or it hasn't) and even a hand held stop watch is good to around 1 sec resolution (mainly the latency in the wetware jelly (<< you!) pressing the stop button)
At a true 2.000000 Hz 1000 pulses takes 500 seconds, so you pulse counting worstcase error is 1/1000 and your time counting 1/500. So the longer you time for, the better your resolution!
Of course, this tells you nothing about jitter and repeatability, just about the average frequency over the counting time period