Whatever O/S the DHCP server is, the lease must be stored on the dhcp client, even if only to know when the lease must be renewed. Where the lease is stored depends of you dhcp client and your O/S.
On OpenBSD the lease history is stored in /var/db/dhclient.lease.interfacename. To see it, you can use 'cat' & tail (to display only the last lease):
benoit@alix [/var/db] sudo cat dhclient.leases.vr2 | tail -14
option subnet-mask nnn.nnn.nnn.nnn;
option routers nnn.nnn.nnn.nnn;
option broadcast-address nnn.nnn.nnn.nnn;
option dhcp-lease-time 604800;
option dhcp-message-type 5;
option dhcp-server-identifier nnn.nnn.nnn.nnn;
renew 3 2009/9/30 08:39:39;
rebind 5 2009/10/2 23:39:39;
expire 6 2009/10/3 20:39:39;
Have a look at the manual pages of your O/S or of the dhcp client running on your box, it must contains this information.