Poor babies. I am glad you found you them and saved them. God bless you.
Telling a kittens sex can be difficult when they are young. Even vets mess it up sometimes. You can usually start telling the sex of kitten around 6 weeks of age.
Both male and female kittens have two openings in the anal area. They are closer together in the female. The one closest to the tail in both sexes is the anus. In a female, the vulva is a vertical slit almost joined to the anus like an upside down exclamation mark. In a male, the tip of the penis is hidden in a small round opening about 1/2 inch below the anus. Testicles in a male kitten usually become obvious when the kitten is about 6 weeks old and clearly visible in an adult male who is not neutered.
Here is a photo to help you