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.
I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.
Death is the messiah. Everything else is irrelevant and arbitrary.