experiment with real populations using Hardy Weinberg

I am helping an IB student making his extended essay in Biology. He wants to prove that a certain population in my country matches better the Hardy Weinberg model than other. The problem he has is that he does not know how to proceed to take the samples in order to get to know the genotypic frequencies. How many individuals should be enough to get a good sample?
In order to determine if the populations are in Hardy Weinberg equilibrium should he sample both the parental generation and the daughter generation or just one of them? I have seen examples of problems in which you just need to know the genotypic frequency of one generation and from there you can infer if the population is at equilibrium. Can you explain to me that case?
Thnaks so much in advance!!!