If your Confluence URL is https://<company>.atlassian.net you are on Confluence Cloud.
If the URL does not have atlassian.net in it, then you are on Server/Data Center.
Another way to check is when you are on your Home dashboard, scroll to the very bottom. if you see a version of the following text with a series of version numbers, you are Server/Data Center.
"Powered by Atlassian Confluence 7.16.0"
We hope this helps but don't hesitate to drop a question below if you have any.