If it's done correctly, like this:
- when do you get to see the meter? Just once per month at billing time?
Whenever you want. Just go to your usage page on the IPS's website and have a look.
- who verifies the meter is accurate?
You can have your own usage monitor on your computer or router if you want
- how are ISPs prevented from abusing the meter? Recall that long ago, laws had to be written to stop phone companies from charging for calls before they were actually answered.
If the ISPs can't charge the user then the only risk is of the ISP lying about usage. Have fixed monthly fee with a cap which cuts off or throttles access to the Internet beyond your ISP once the cap has been exceeded. Customer can pay to top up, ideally for slightly less than the price/GB of the basic cap. Customers can also configure an automatic top-up charge for convenience if they wish.
- how are bytes being counted? Bytes are not counted like phone minutes. Packets are re-transmitted out of necessity. Do they count twice?
There are plenty of perfectly valid answers. Most policies are fine as long as they are clearly-defined.