Determine the type of fuel used -- it may be gas, liquid or solid.
Calculate the caloric value of the fuel and its flow rate. For gas fuel, these numbers can be obtained from the gas supplier. For liquid fuels such as fuel oil, a meter will need to be installed at the combustion source. For solid fuels like coal, it is difficult to assess without sending a sample to a lab for analysis. This is the boiler's input.
Install a flow rate meter to determine the boiler's output. Consult the manufacturer's manual to determine if the output rate need be corrected for temperature or pressure.
Determine the boiler's efficiency. Divide the boiler's output by the boiler's input. Multiply by 100 to get the percent efficiency rate. Assess results.