SQL Server built-In monitoring components

First, It should have been determined on where/what to monitor for the current situation. right ? Yes!

And, select an appropriate tools to proceed with. It can be either from Windows Itself Or SQL Server specific

Windows Monitoring Tools:

• System Monitor
• Performance Monitor
• Task Manager
• TypePerf

SQL Server Tools:

• Activity Monitor
• Database Console Commands (DBCC)
• Distributed Replay Utility
• Dynamic Management Views and Functions
• Extended Events
• Functions
• SQL Server Management Studio Graphical Showplan
• SQL Server Profiler
• SQL Trace
• System Stored procedures
• Trace flags

Note: 

• SQL Trace and SQL Server Profiler are deprecated. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.
• Use Extended Events instead

Source: https://docs.microsoft.com/en-us/