How to know per pool available leases ?

[Leandro]

Hi guys, I would like to know witch isc dhcpd feature can I use to 
notice if some pool is getting without leases.
Reading the documentation founded the:
log-threshold-high \percentage\
log-threshold-low \percentage\
According to the doc, those  lines make the daemon  write to the log 
when the pool usage reach the defined low and high  thresholds.
I have been trying to make them work but could not succeed yet.
So:

Is someone using this feature ? Where should those lines be defined ? 
Can share an example?
Is there another way ? May be through omshell?

btw: Already triyed dhcpd-pools

Regards.
Leandro.