Idiotic question, so here's an idiotic reply:
jar tf /path/to/java/lib/rt.jar |\
egrep -v '^(META-INF|com/sun|sun)' |\
sed -e 's/\/[^/]*$//' |\
sort -u |\
sed -e 's;/;.;g' |\
wc -l
And if you change the last wc -l to more, you'll see the actual package names.