People born with PKU have approximately a 1/100 chance of having a child who also has PKU. I do know PKU adults who also have children with PKU, but I also know plenty that do not have PKU children.
Clenk is right that there is a blood test to determine whether or not someone is a carrier for PKU. However, my understanding is that the test is expensive. It would require a doctor to search for the PKU mutation. The problem with this is that there are over 500 PKU mutations, which as I'm sure you could imagine would make it difficult to know exactly what mutation the doctor needs to look for.
As for getting your child's Newborn Screening results back fast talk to your dietitian. When my mom had her last two children she asked my dietitian to contact the lab that the results are sent to. My dietitian kept on the ball, and we had the results of my sister and brothers Newborn Screening by 4 days old and 3 days old. As it turned out my sister did have PKU, and she started taking Phenex 1 that day, because our dietitian had given us an extra case of it just in case (this also saved my Mom a trip to the hospital to pick some up). Even if the baby has PKU she should be able to breast feed if she wants. My Mom could not breast feed me, because that was the belief nineteen years ago, but she did breast feed my two younger sisters that have PKU, and they are doing great today.
I hope this helps, and congrats on the new baby. Good luck :).