What are the measurements that will evaluate you if you're doing your job correctly?

Physical proof that your doing your job correctly is the quality of your product or service. 

I believe there are two way to view your question , job in general, or your job maintaining the system to achieve "good job" (looking at the question tag, Desktop Support, System Admin)

1. Job in General :

Our company conduct2 review per year, one with manager, who provide perfomance metric, target for each department and from company in general. The metric in this case include teamwork : how much we conform or exceed customer SLA, how much cost saving we make  etc, individual metric include new skillset acquire during half yearly, and some individual target metric. We do also have anonymous peer review where we give review on each colleague, even evaluate how our manager perform. I find myself having a clear target and objective of what to do with these method. Half yearly is a good timeframe for this.

2. As a System Admin - we uses tools such as witbe for perfomance/SLA monitoring. Witbe uses script to perform automation on our system such as login/ page load/object upload and download/ transcoding etc . The timeout/performace of each activity is discussed and agreed with customer so that its within our resources to achieve. Both customer and us will receive notification for each warning or breach and we act quickly for each breach. Of course no system is100% perfect so in our experience, communication with customer, being transparent with them is the best measure here when there is a breach.

We do also monitor performance of our system using tools such as ganglia/nagios. We react to the system before it affect and give impact on our SLA. 

This will show by stability of your company system also by  performance and employee's satisfaction also system effectiveness