I would suggest you start by reading the overview - is (or should be) documented here:
But how to debug/trace upstart is described in the upstart documentation:
You could also get a list of all events using command line:
$ cd /build/atlas/etc/init
$ grep -Rh -e "^start on" -e "^stop on" | sed -e 's/start on //' -e 's/stop on //' -e 's/stopping //' -e 's/starting //' -e 's/started //' -e 's/stopped //' -e 's/ or / /' -e 's/ and / /' | sort | uniq -c | sort -n
There might some be built-in events that aren't referenced by Chrome OS scripts.