Well I cannot speak for all of the HVAC guys out there, but my company has a "full service" check that we do for our bi-annual service. We do a visual inspection of the AHU (all components looking for signs of deterioration, heat, moisture, excessive dirt build-up, etc.), we check the venting and exposed ducts (for leaks, corrosion, and any other damage), we check the gas pressure entering and exiting the gas valve (gas only of course ), we check the amp draw of all motors and the entire system, we do a combustion analysis of the system to ensure proper efficiency, change the filters, we check the "delta T" to ensure proper airflow, and then we scrutinize anything that doesn't meet specs. We do the same thing in the spring for the AC side of the system (visual inspection, amp draw, "delta T", refrigerant pressures etc. etc.