In most cases, it's impossible to determine the integrity of a heat exchanger without pulling the blower and burners. (DO NOT EVEN ATTEMPT!) It's possible to see approximetely 1/3rd of the heat exchanger on natural draft furnaces without taking anything apart. (aside from removing the cover)
You should get it checked every year.
The likelyhood of finding cracks in a 26 year old unit is extremely high - plan on getting it replaced.
General public's attitude towards our energy predicament:"I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"
How would a homeowner know if their Heat Exchanger has gone bad (a.k.a. Cracked)??
I have a 26 year old furnace. Still works fine.
26 years old is WAY up there in age for a furnace. There is a very high probability of a heat exchanger crack or other type of dangerous situation with your furnace. (Gas flex to the furnace may even be unsafe) Contact a professional to do a thorough inspection on the unit.