For a regular sinusoidal function the period is the distance between two points that have the same value f(x)and the same slope. You describe it as "skipping one," which is OK, as the point being skipped has opposite slope of the first point. So yes - you can choose successive highs (or lows) - the slope is 0 at both points so it's easy - or you can pick two successive points where f(x)= 0 and slope is equal (i.e. skipping the intervening point where the slope is opposite sign), or you can choose any value for x, note what f(x) and the slope are, then look for the next point with the same value of f(x) and slope.