Frequency and friction
A good candidate happens often, consumes time and follows sufficiently clear rules. The more ambiguous it is, the more important human review becomes.
Data and exceptions
Before building, locate the input information, its quality and the exceptions that change the decision. Automation without this reading only moves the problem.
Verifiable value
The first metric can be simple: steps removed, time recovered, errors avoided or response speed. The system should prove usefulness before scaling.