You can use the CodeWatch debugger to verify various states in a PL/I program. The debugger can stop program execution and send a notice when it detects a condition that may trigger an ON-unit. This lets you to investigate the program's state prior to the execution of the ON-unit. You can then set up breakpoints as needed before continuing program execution. Sending notifications using ON-unit detection can be done using either GUI or Command-Line CodeWatch.
You can activate or deactivate for all supported ON-units.