Don’t know if this really answers your question or not, but I inspected a 34 year old condo earlier this year that had the original furnace. I knew before the inspection that the client had also hired an HVAC tech due to the age of the furnace and that he was scheduled to be there about an hour after I arrived. The first thing I did was fire up the furnace to see if I could detect any issues using methods similar to yours. It seemed to be fine as far as I could tell. The only thing that I noted was some corrosion on the joints of the flue.
The HVAC guy showed up, probably about 65 years old. He pulled the cover off, reached into the chamber and said that the heat exchanger was cracked. I asked him how he knew that. He said that’s what 35 years in the HVAC business gets you. He knows what models have what problems. He stated that the corrosion on the flue was also an indication of the cracked heat exchanger. He wasn’t cocky or gave any indication that I wasn’t a competent inspector, he’s just done this for years and knows exactly what he’s doing and what to look for. I ask him if it was leaking CO into the home, and he indicated that it probably wasn’t an issue. I don’t remember the details, but he stated that the furnace would stop working before the CO levels would pose a safety concern. I talked with him for probably another 30 minutes just because he was so humble and a great source of information.
After that encounter, I changed my verbiage in my reports. No matter what my findings are regarding a furnace, if it’s over 20 years old, I recommend hiring an HVAC tech for further evaluation. I have always recommended contracting with an HVAC company for annual service, but he really opened my eyes. I’m not quick to refer systems out to contractors, but we’re talking thousands of dollars that she was probably able to negotiate with sellers.
As far as borescopes, tearing into a furnace, etc. I wouldn’t recommend it unless you really know what you’re inspecting. I know that you don’t want to look unprofessional, but think of how it would look if a clients’ furnace quit working after they moved in and they discovered from an HVAC tech that there were issues with the furnace that you didn’t find.
Best of luck!