If Debian is to migrate to upstart on Linux, I expect some init.d scripts to migrate (some of) their operations to upstart job while keeping the init.d for hurd and kfreebsd. The packages with such needs will need a way to get their init.d scripts to behave differently when used with sysvinit and with upstart. Because of this, I had a look at the environment variables set when a init.d script is running under upstart, and when it is not.
With upstart, I notice these environment variables are set when a script is started from rcS.d/ (ignoring some irrelevant ones like COLUMNS):
DEFAULT_RUNLEVEL=2 previous=N PREVLEVEL= RUNLEVEL= runlevel=S UPSTART_EVENTS=startup UPSTART_INSTANCE= UPSTART_JOB=rc-sysinit
With sysvinit, these environment variables are set for the same script.
INIT_VERSION=sysvinit-2.88 previous=N PREVLEVEL=N RUNLEVEL=S runlevel=S
The RUNLEVEL and PREVLEVEL environment variables passed on from sysvinit are not set by upstart. Not sure if it is intentional or not to not be compatible with sysvinit in this regard.
For scripts needing to behave differently when upstart is used, looking for the UPSTART_JOB environment variable seem to be a good choice.