Every codebase is fine while the people who built it are still there. The question is what happens when they are not, and it is answerable before you find out.
1. Can a new engineer get a working local environment in one command?
2. Does a full test run finish in under ten minutes?
3. Are your conventions enforced in CI, or written down?
4. Is there a written record of decisions that look wrong but are deliberate?
5. How many people understand the billing or payment code?
Indicative. Measures how expensive it would be for someone new to take over.