How do I set dhclient debug info on?

dhcpdump is the tool of your choice.

Just use dhcpdump -i fxp0.

I found more information at http://www.freebsdonline.com/content/view/713/524/

From the manpage:

It can be made to emit verbose messages displaying the startup sequence events until it has acquired an address by supplying the -v command line argument.

Type:

dhclient -v eth0

...where eth0 is your network interface.