Only thing I came think of would be to do something like:
Create an IPv4ACL and set it to allow from the wireless client subnet *to* the wireless client subnet and setup the ACL to log (which should then cause any traffic detected going from one wireless client on that subnet to another wireless client on that same subnet to get log in the event viewer) and then apply this IPv4ACL to one of the APs.
This is really a 'kludgey' way to try to test this though.
As Daren mentioned, probably best to enable the option and then run the 'service pktcap on drop' command and look at the traffic.
Or...if you don't want to YET enable the no client-2-client feature but just want to see if it's happening...then you could still run that command, but look for Source and Destination addresses of traffic that belong the the DHCP pool for the SSID you want to monitor. If you see traffic that is going back and forth between wireless clients on the subnet, then there *is* client-2-client traffic happening.
I don't see any other way to checking this w/o it the process getting more complicated.