sysconfig -ca will show you what part is in each slot, and by elimination you can tell what slots are free, in my lab slot 3 and 4 are open. This command is great for knowing what the NetApp part number for your cards is as well!
Assuming a Clustered OnTap system
system node run -node <NODENAME> -command sysconfig -ca
sysconfig: slot 5 OK: X1936A: Performance Acceleration Module I
sysconfig: slot 6 OK: X1107A: Chelsio S320E 2x10G NIC