if you set up the "status" directive in your server.conf you have a text file constantly updated with active clients/connections with some info.
see "status file" here
http://openvpn.net/index.php/open-source/documentation/howto.html
is something like:
# cat openvpn-status.log
OpenVPN CLIENT LIST
Updated,Mon Jun 3 17:10:52 2013
Common Name,Real Address,Bytes Received,Bytes Sent,Connected Since
client_example,xxx.yyy.zzz.www:37058,5371,7234,Mon Jun 3 17:10:35 2013
ROUTING TABLE
Virtual Address,Common Name,Real Address,Last Ref
10.8.0.5,client_xxx,192.168.x.y:37058,Mon Jun 3 17:10:40 2013
GLOBAL STATS
Max bcast/mcast queue length,0
END
keep in mind it is not real time but updated every few seconds...
another useful tool is the management interface (see just after "status file" in the howto above)
Marco