In article <25267.945690391@coconut.itojun.org>,  <itojun@iijlab.net> wrote:
>	- use /sbin/ifconfig and some other tools.  I may need grep, head, tail
>	  and some other things but they are in /usr.

A little cheesy but:

ifconfig lo0 inet6 > /dev/null 2>&1
if [ $? != 0 ]
then
	echo "No ipv6 support"
else
	echo "Found ipv6 support"
fi

christos