If they use any table or any of the features where there are a group of objects (pages) that have different status or properties, they are using databases.
It's pretty much something you accidentally create and use in notion without knowing unless all you are doing is writing documents.
Anything needs to be shown on a table, calendar, etc. is a database. It doesn't matter if user deliberately uses it or not. One can have a look at all the Notion's templates (https://www.notion.so/templates). All featured templates have databases. I also looked at all categories and their front pages filled with templates /w databases. Maybe 1 per category without it.
It's pretty much something you accidentally create and use in notion without knowing unless all you are doing is writing documents.