How to inspect high-efficiency furnace venting

High-efficiency furnaces use PVC pipes to vent the exhaust gases out of the home, and manufacturers are very specific about how the installation should be done. As a home inspector, how far should I go to make sure the vent is properly installed? Of course, I'd report on a vent that wasn't vented to the outdoors. But what about a vent that terminates too close to an openable window? Maybe, right? And what about a vent that has too many elbows? I think this is where a lot of home inspectors start to shake their heads and say that this goes beyond our standard of practice... but why?
I won't tell you exactly what a home inspector should or shouldn't report on, but I will cover some of the most common defects that we come across as home inspectors.