A few criteria/questions that can help with this.
- Is it perceived that the process design and flow itself is part of the issue?
- Is there a problem with lead time or quality of the output?
- Is the solution or root cause unknown?
These are 3 questions to help get you started. For a more extensive list, we have a project selection matrix that can help. https://goleansixsigma.com/project-screener/