Both of my hens have large, cupped wattles, and are definitely females. I would not use that method to even guess at the sex of guineas, as it would just be a guess.
As I said it is not 100% accurate. None of my hens have the large cupped wattles but I did have males with the small flat wattles. I suspect that wattle size may also be somewhat determined by the color of the guineas since all of my blue male guineas seem to have smaller wattles than the other colors.