A far better scheme than trying to use the ALE signal would be to allocate a different ordinary port pin as an output. Put into your code someplace some logic that will output a narrow trigger pulse on this pin from time to time. (For example in main loop of program have the pulse go out every N times around the main loop where N is divided down by a software counter variable).

Connect this port pin to some type of re-triggerable monostable circuit that has its period programmed about 2X or 3X the normal output frequency of the port pin pulse. The LED is then driven in a manner to be ON whenever the monostable circuit is triggered. If the LED goes off then the main loop has stopped cycling normally and if the LED flashes then the main loop is operating erratically.

There are several alternatives for the monostable circuit but you could check out something like a 74HC123 chip.

Michael Karas