Depends if anything is using it - nothing should be using it (AFAIK), like no services should be running as a domain administrator, but only you can say for sure in your environment.
I think your options are:
- Run
exec sp_who2 in a new query in SQL Management Studio, that will show you who is logged in right now.
- Run Tools -> SQL Profiler from SQL Management Studio, logging batch transactions, and watch some transactions during the day, that will show you which accounts login frequently.
- Turn on SQL login auditing and watch the logs for a while to see which accounts login.
But to be certain, you'll have to audit the things which need to connect to SQL and identify what accounts they use use. That's probably a good thing to do anyway if you don't already know and have no documentation/records to check.
Don't forget to look for SQL Server Reporting Services, any backup system, any scheduled tasks or agent tasks.
It would also be a reasonable time to make some monitoring system or PowerShell script checks of functionality - e.g. run a HTTP(s) request to every website and make sure they (a) return a 200 response and (b) have the right content, that you have succeeding backups. Then you can verify things work after making any changes.
Then reset the SQL sa account password.
Or, reset the sa account and see what breaks - you know the current password, so you can set it back in a hurry if you need to. I didn't suggest this.
