How do you know the sex of kittens?

Hello I have 4 kittens all strays that I took in. They are a month old and I'm wanting to know the sex of them. I can definatly tell there is one boy and one girl,but I'm not sure about the last two. (They each have three holes) I'm guessing they are boys but the ballsac isn't really defined yet. So if anyone could help me out that would be great. Thank you!

Maybe this diagram will help:
http://www.peteducation.com/article....&articleid=923

the girls have two dots that are closer together and the boys are further apart. Sometimes the girls looks like a slit and the boys looks more like a dot.

Female.

The genital opening is slit shaped and quite close to the anus.

Male.

The genital opening is round. If you gently press under the opening a penis may be extruded. The distance between the anus and the genital opening is relatively greater than in a female.

Hope that helps, Good Luck!