determining gender of new born rats

Question
I wan't to buy two one month old rats from a pet store. How can I besure thay are both femele? The owner does not now. He said thay have to be two to three months old to tell. can you help me:]

Answer
Wait until they are two months old. They shouldn't be weaned until about 5-6 weeks anyway. However, it does worry me that the owner can't tell until 2-3 months because by 6 weeks they are capable of getting pregnant and need to be separated... In any case, here's some good sexing information:
http://www.ratz.co.uk/sexingrats.html