Talk to the people who do the work

What helps on a first call

• What the system does — in business terms, not technical ones.
• What it runs on, as far as anyone knows. “A server in the closet” is a legitimate answer and a common one.
• What triggered this — failing hardware, an auditor, a retirement, a version that stopped being supported.
• What must not stop during any migration.

If you do not have those answers, that is fine. Establishing them is the first thing we would do anyway.