If you can share the brand and model # of the furnace, there should be someone that can give you a link to the service manual.
When a DSI furnace is first called by the thermostat to start the heating sequence, you should be able to hear a light snapping sound. This is the spark jumping across the electrode. There may be a removable panel next to the furnace that allows for it to be accessed. Also, many have an exterior door that allows access. If the area surrounding the furnace is dark or not well lit, careful observation should show the spark of the electrode.
There are also different parts of the furnace that all have to operate in order for the burner flame to light, principally the sail switch and high limit switch.
