The only theory that I've heard that even comes close to being accurate is the one about their wattles. Both male and female guineas have rather prominent wattles. This theory says that the females have small wattles that point backwards, like the white female guinea in the top picture.
The males are said to have larger wattles that fold over and point down, like the male pied Guinea in the center. Obviously, this sometimes holds true. There seems to be a good amount of Guineas that look like the third picture though. As you can see, one wattle is small and points backwards, the other is larger and folds over and points down.
Now, I'm not trying to break this theory wide open because after all, I don't have a definitive answer. I do know that the majority of Guineas I have like with 2 different wattles are female. They do lay eggs. I have hatched their eggs with no problem. I've often wondered if there was some genetic issue, but they have been as healthy as any other Guinea and lived just as long. (While the Guineas in the pictures are different colors, color isn't relevant they just happened to be the only ones that wanted to stand still for me today!) Honestly, I'm not sure what's going on and I'm pretty sure it really doesn't matter unless you're using this theory to purchase birds, in which case I wouldn't rely completely on this method.
The only method you can rely on is the 'buck-wheat' test. The female makes a 2 syllable call that sounds like she's screaming (and I do mean SCREAMING) 'buck-wheat buck-wheat'. The male call is a 1 syllable call which sorta sounds like 'chi chi chi' and isn't quite as loud. Usually. Here's where it gets difficult....the female can make both calls, the male cannot. So, a 'chi chi chi' doesn't always mean you have a male, but a 'buck-wheat' always means a female. Luckily, I've got it on video for you.