The most comon way to detect a crack in a heat exchanger is with a visual inspection, removal of the burner assembly and the use of a mirror and flash light.
Or a smoke bomb test. A special little smoke bomb is placed in the burner area with the burners OFF. The blower is runnig and you light the smoke bomb, the smoke will draft up the flue vent or if a crack or rust hole is present then the smoke will get sucked into the air stream and come out of the duct work.
Other than that using a carbon monoxide detector to see what the CO levels of the furnace are and comparing them to the supply air of the furnace to verify that there is no CO present in the air stream.
The best and most accurate way is to contact a local Heating and Air Conditioning specialist or your local gas company should come out and perform a CO test on the furnace.
Good Luck and God BLess