one thing techs should be checking for is temperature rise. this differential indicates whether there is an air flow issue that could cause premature deterioration of the furnace heat exchanger as well as possibly detecting a restriction at the surface of the evaporator. air flowing across the furnace to pick up heat and carry it into your house also has to travel through the fins of the evaporator. the acceptable rise is published on the name plate of the furnace.
the older the furnace, the more likely a crack in the heat exchanger. over the last 20 years, there have been great strides in technology improvement. changes in alloys, physical design and electronics have made furnaces more 'failsafe' and less susceptible to cracks and other heat related defects. units manufactured in the last 5 years are safest, but are not designed for excess heat buildup. in most cases, if this problem exists, heat limit sensors strategically located in the unit will detect the problem and after repeated incidents of detection, will lock the system out, which of course will result in failure detected by you when your heat doesn't work.
any system over 10 years of age should be tested for cracks. as mentioned by another contributor, though a necessary safety inspection, the process can be used by unscrupulous people to scam you into buying a new furnace before you need to. the method of checking can include an electronic detector used to determine if there is carbon monoxide in your air stream. units 15 years and older may have a heat exchanger design that requires removing the burners and/or blower assembly to inspect the heat exchanger for cracks.
other inspection items should include testing for gas leaks, running the system through its cycle to test sequence of operation, testing safety controls for proper operation and verifying proper thermostat operation. beware of companies that hire sub-contractors to do service. not all, but some contractors and their sub-contractors make their money through sales and gouging. find out fees up front and as was mentioned, request a complete rundown on checklist items as well as a complete report of findings. you can always get a second and third opinion.