Like Kevin said, you can sometimes tell be their wattles; but not all of the time.
Here is a picture of my female Slate Hen.
This is my Slate male.
You can see how the hen's wattles are flatter and are not hanging down as low. And the male's are cupped, larger and hanging lower.
The best, and sure way to sex guineas is to listen to their call. The hens will make a two syllable sound that sounds like, buck-wheat -buck-wheat buck-wheat. The males can only make a 1 syllable noise that sounds like, chi chi chi chi.
Oh yeah, hens will also have a more lumpy body. They will have what is called a 'bustle' and males will have a more round body and stand tall, while hens like to stay close to the ground.
Check out this thread to see pictures and get more info. Click
Like the others said, the voice is the only sure way to tell. Once they are fully mature and you spend some time with then you will be able to spot who is who.
The males wattles tend to form in a bowl shape. The females are louder then the males and tend to make a "buck wheat" call. the males just say "Buck". The females will go on and on more than the males.
I have 3 females and 2 males. The females are smaller and say 'Come back' 2 syllable- whereas the males don't. They all screech and chatter though lol.