Call them with someting like this:
label debian-8-preseed
menu label debian-8-preseed
kernel os/debian/8/debian-installer/amd64/linux
append vga=normal initrd=os/debian/8/debian-installer/amd64/initrd.gz auto=true url=http://www.stacken.kth.se/%7ensg/preseed/debian8-nsg.cfg DEBCONF_DEBUG=developer
DEBCONF_DEBUG=developer will add a lot of extra debug info to tty4 and syslog.