Now my question: Since my background is in accounting and auditing with a touch of fraud investigation, how do we consumers really know that our electrical (or gas) meters are, indeed, accurate? What controls are in place at utilities to ensure the accuracy of their meters? I know of no testing service or equipment that we can use to verify over a lengthy period of time that what the meter displays is accurate to a very small number (maybe a milliwatt-hour?)
Put it this way. The meters have to be accurate because the law says so, and they are. They are tested by public regulators or their delegates. Also, keep in mind that PG&E and all California investor-owned utilities are decoupled. This means, they do not make more money the more energy they sell. That's right, read that sentence again.

They are actually incentivized to operate efficiently and can make an authorized return on the installed capital base.
How much electricity PG&E sells, for example, does not impact the profit line at all. This is why PG&E is so proactive about things like solar or energy efficiency audits or giving away free LED light bulbs at community events. A company that makes more money by selling more power, would never do that. This law that is enacted in California, has led to our state having a flat per-capita consumption curve for the last few decades while the nation kept using more energy per capita. But I digress.
If I want to audit the meter data, I can do so via my SunPower app. I have solar on my roof and SunPower installs a power monitor that gives you gross and net. It graphs it out really nicely too.

So there are ways. And in my experience as one customer, the meters are accurate.
- K