One bit of advice I can give about sexing young rats is to not pick them up by the tail to do so. Males pull it all in when you do so and to the inexperienced eye they'll look just like females.

Basically you're looking at the urethra bump; on females it will be right up next to the anus, and on males you'll see a defined gap between the urethra and the anus. What you should probably do is just pick up two at a time and look pretty closely at each one side by side. You'll see what the link Tsandford provided was talking about eventually and once that clicks sexing becomes really easy.

Also yes, separate them out at weaning.