What it would fix, what it quietly does not include, and whether the number is a plan or a guess.
Questions built from your real system, and a way to read the answers without being able to write the code yourself.
When you are told something is impossible, or necessary, or urgent, and you have no way to tell which.
What you need to get back from someone who is leaving, and how to check that you actually got it.
Pick the level of involvement that fits, describe your product, and we begin there.