Use a program like top
(on Solaris), the Windows Performance Monitor, or a more sophisticated tool to keep track of information like:
CPU utilization
paging activity
disk I/O utilization
network I/O utilization
A well-performing site will have high CPU utilization when the site is achieving its maximum throughput and will not be doing any paging. A site with high I/O activity and low CPU utilization has some I/O bottleneck.