Hardy-Weinberg assumes that there is no selection, gene flow, or genetic drift, and that the population is infinitely large. Another main principle of HW is that average allele frequencies will not change from one generation to the next. If we know HW equilibrium is taking place, then the population is not "evolving," which in this context means that the allele frequencies are not changing. Therefore, if a population has allele frequencies of p=0.5, q=0.5, then we can assume the parental generation had the same frequencies. It doesn't matter what the frequencies are, as long as there is HW, they will not change.
Without knowing a population is within HW however, you can tell if a population is evolving without parental info by observing the 2nd generation (F2) allele frequencies to see if they differ from the F1 generation.
Hope I helped, let me know if anything doesn't make sense or if you have any questions feel free to message me.