Where I work we do a penatration test. This requires you to remove the blower. Inspect the heat exchanger before, look for protenial spots. Then squirt water on the Heat Exchanger form the bottom. Get it wet as possible. Then inspect again, if wets form on inside of heat exchanger you got cracks. Another test is the smoke test. They make a smoe bomb that you light off in your heat exchanger then turn on the blower and if you get any smoke out of vents you got cracks in your heat exchanger.
What other way is there to check for bad exchanger
besides a visual inspection.
I use a scope but the scope still cannot get in some places so in that case I either pull the blower whether it is downflow or upflow and look inside with a flash light or if upflow and if I can take the spacer from the a coil I look down that way also then do what twilli does.