To open Resource Monitor go to Window menu on macOS menu bar when your virtual machine is active > select Show Resource Monitor:
Alternatively Resource Monitor can be launched using the same macOS menu bar when Parallels Desktop Control Center is in focus.
Resource Monitor will appear and stay on top of all open windows. Since Parallels Desktop 14 for Mac it lists all running virtual machines:
Resources used by the Mac side are shown using white color. Each running virtual machine has its own color which corresponds to the color of the name of the virtual machine in Resource Monitor.