I've been in the plumbing and heating business as a licensed contractor for over ten years, a plumber for almost twenty years .
One essential tool for any contractor servicing AND installing gas or oil fired appliances like furnaces, tankless water heaters, tank water heaters and boilers is a combustion analyzer; yet I still meet contractors who do not own/use one. Are you one of them? How is it possible to know if an appliance is operating properly? Wait, I'll answer that, ITS NOT POSSIBLE .
Some municipalities where we do business in central MN require combustion testing upon completion of permit inspection, I would prefer every city and town enforced this rule for the sake of quality work and safety of our customers. Not to forget energy conservation .
If you're curious about tools like combustion analyzers, digital manometers or other test instruments there are many resources available online for training and buying guides. You can start on my website (link in bio) or do a simple Google search.