Inspect the heat exchanger within the furnace for cracks, openings or excessive corrosion. Typically the blower will need to be removed and the inspection performed through the blower compartment with mirrors and cameras. While visibly inspecting the heat exchanger you will want to look for the following possible issues: Cracked cells, missing eyelets, corrosion, and holes.
90% furnaces from the mid 1980s through the mid 2000s are known to have the secondary HEX fail. The plastic lining in the interior warps away from the steel portion of the HEX and the corrosive condensate eats away at the coil from the inside. The blower must be removed in order to inspect the secondary heat exchanger. Often times there is a visible leak.