To check MSSQL Database server instance Name in Plesk, Please follow the following steps :-
1) Login to Plesk control panel.

2) Click on Websites & Domains.

3) Click on Databases.

4) There you can see Instance name under "Database Server" as per marked in RED

If your database MSSQL so you see .\MSSQLSERVER2008 OR .\SQLSERVER
