Heat exchanger can crack for various reasons and should be checked every time furnace is repaired or maintained.

Visually check heat exchanger for cracks

You will find most cracks form near the back and where the heat exchanger metal bends, Since this is where it will be weakest due manufacturer weakening the metal by bending it.
Watch the flames as the furnace fan starts if you see the flames are blowing around or out the front of the burners then you will need to take a closer look at the heat exchanger
Water test

This test worked best on the old Lennox G8 and would find cracks or pinholes on the heat exchanger that sometime could not be seen.
If there is no air conditioning then it will be easy to access above the heat exchanger through the humidifier. Once the heat exchanger cools down spray water on the heat exchanger from the top insuring full coverage and being careful of the electronics and motors. Then watch inside the burners for water to seep through, A good heat exchanger will not let moisture into burner chamber
Using carbon monoxide detector

Test for carbon monoxide at return air as well at supply air and compare the two readings or test at supply air with burners on then off making sure furnace is not back drafting and compare.
If you take reading once and only at supply air you will not be 100 percent sure the problem is with the heat exchanger. You are testing the accumulation of carbon monoxide over a long period of time from other sources like the furnace or hot water tank back drafting also it could be from outside sources like a vehicles near the intake air.
Reasons for cracks

If the furnace does not have enough combustion air it will cause the furnace to run hotter then designed. Then with more expansion and contraction in the heat exchanger it causes cracks. The gas pressure must be set as per furnace name plate derated for location altitude if too much gas heat exchanger will run too hot.Blower speeds must be checked by comparing furnace temperature difference to name plate rating if running too hot increase fan speed and recheck temperature supply air minus return air.
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Any crack in a heat exchanger is dangerous and the heat exchanger must be replaced when crack is found

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