I am reposting the response that WOT gave re: diagnostic tool. I am sure some of you guys that get a Volt will want to dive more deeply into the data that is stored on the Volt's computer.
Here is WOT's response:
This will not be easily accomplished with a mere scan-gage or CAN appliance without detailed information on the GM Parameter ID structure. Most of the "generic" devices out there (including scan-gages) will monitor and display only basic OBDII and engine related items, many of which are SAE mandated. However after that things get a bit more proprietary with respect to GM's own "interpretation and implementation" of CAN known as GMLAN.
The EREV specific PIDs you are looking for are very likely spread out amongst the 28 networked modules the Volt employs to comprise it's GMLAN network/s.
The only tool solution guaranteed to delve into it all is the genuine article, the Multiple Diagnostic Interface (MDI) produced by Bosch for GM.
The MDI combined with access/license subscription to GM's Global Diagnostic System software, (a GM version of Bosch ETAS) would IMO be the only reliable method of getting this data.