Pam Mooney shows how you can discover and document limits for a SQL Server instance:
Having taken steps to map your database applications to the databases and address your security and backups, you need to turn your attention to your server’s limits.
What do I mean by limits? Certainly, this is an allusion to how you will monitor your server drive capacity, but I also mean how you will track resource limits, such as latency, CPU, memory, and wait stats. Understanding all of these terms, what normal values are for your server, and what to do to help if the values are not normal, will help to keep your servers as healthy as possible.
These measures are big, in-depth topics in and of themselves. This will only serve to get you started. Links to more in-depth resources are included with each topic, and you will doubtless find others as you progress through your career.
Click through for the process.