Same way as after StackOverflow. A combination of thorough logging throughout the codebase at various levels of detail, one off print statements, then once it'

Same way as after StackOverflow. A combination of thorough logging throughout the codebase at various levels of detail, one off print statements, then once it's narrowed down to a specific chunk of code reading it intently and finally if that doesn't work fire up a debugger and step through it.