Before you can check if any system is performing optimally, you first need to know what you expect it to be doing. Ultimately, this is to satisfy the business requirements for which the application was deployed.
These requirements should be expanded further and form part of the Service Level Agreement (SLA), which defines the nature of the service that the system and application is expected to deliver. Then you need to define a number of Key Performance Indicators (KPIs).
The SLA is an end-user oriented document, using standard business terminology. KPIs are specific definitions of quantities to be measured, that together show that the SLA has been met. The SLA mentions the general targets of the system.
The KPIs define exactly what to measure, how to measure it, and the allowed values. The KPIs are unambiguous so that there are no disagreements over what the target is, what is being measured, and what the system is achieving.