I just went through this 4 months ago with my 2nd pregnancy. My daughter, Alysa, is 25 months (diagnosed at 18months).
The test they did for me, and I believe its pretty standard, is through amnio. I was scared at first of the thought of them putting a needle in my belly. I heard horror stories about them being huge but honestly it wasnt bad.
For me they hooked me up the ultrasound machine so they could see where baby was and then the doctor came in and put the needle in my belly (it felt like it was going through my belly button but it was about where your elastic to your underwear would sit). They took a few viles of amniotic fluid and then I was done. They then test the amnio fluid against the same mutated genes that your first child would have.
They told me there is a slight risk of miscarriage (about 1 in 200) but you could have that risk walking down the stairs. We decided to go through with the amnio because we wanted to prepare for the baby and also to find out from a 100% accurate source what we were having (another girl).
I should warn you though that it could come back negative and your baby actually be CF if, and only if, it has different mutated genes then your first child. This would ONLY happen if you, or your husband, actually had CF and didnt know it. The doctor gave us a 95% accuracy rate and we thought that was pretty good in our books.
Turns out the one we are expecting DOES have CF so we are going to have two now. At first, I will be honest, my heart broke but then I looked at with a silver lining in the fact that now my girls can support each other in a way that my husband and I cant. They can do their treatments together and actually have someone to talk to who knows what they are going through.
Best of luck to you and please let us know what you decide to do.