Good site, not the way I do it but I guess it works.
My way is a bit simpler, at least I feel that way. Pick a rat up, I always do it by the tail, wich makes the males draw their scrotum in. But if you hold it, and GENTLY apply some preassure on the stomach it will pop right back out, if nothing pops out you got a female. I have sexed rats as small as fuzzies with this method.
It's fast and easy and the best way I can imagine for me.