DHCP PROTOCOL

My question is: If DHCP auto assigns TCP/IP addresses to workstations how does one determine what IP address a work station has been “leased” at any given time and to what benefit is it to have “dynamic” IP addressing? I have a client who is workingon DHCP and yet when i view the TCP/IP properties at any work station there is no IP address. How does one “ping” the nic card?