This is not a dumb question, it’s a great question and I’m glad that you’re asking.

Human processes

Make sure that you’ve reviewed all documentation, talked to the greybeards, and have sign-off from someone from the business.

Technical processes

Get a complete backup; mark the media for long-term archival. Run a connection monitor or packet sniffer for a period of time to see what connections are still being made. Inspect the services to see if anything sounds important/familiar.

Cutting the cord

Better idea than powering off – unplug the network cable for a few days. If it’s an old physical machine, you don’t want to risk the situation where you need to power it back up but the disk spindles are frozen. Leave them spinning.

Source of authority – I spent over a year decommissioning old servers for a Fortune 25 pharma company. This was the process, and it worked.