Hello @wylbur and welcome to the forum.
Are you talking about mdraid or “hardware” RAID? The latter doesn’t make any sense on modern systems and it is very vendor-dependent.
In the case of mdraid, I’m not sure if a live system starts automatically what’s needed to detect the arrays, I haven’t run a live installation in a while. Regardless, you should be able to use mdadm --detail --scan before you start the installer. With that, mdadm will look for array information in all of the partitions listed in /proc/partitions. If for whatever reason this doesn’t work, you can assemble your sets with something like mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1 (replace with what’s appropriate for your configuration).
At this point you can start the installer and you should be able to see your md devices in the disk selection dialog.
A long time ago, several months after I had set up a system, I realized that /etc/mdadm.conf was never created and as a result the mdmonitor service failed to start. My RAID10 arrays were happily running all this time. All it took was mdadm --detail --scan >> /etc/mdadm.conf followed by systemctl restart mdmonitor.service. Perhaps I had hit a glitch at the time (it could have been F14 or F17, I don’t remember) and this is no longer necessary.