Here are a few command line tools that will help you estimate your system usage and performance.
There is a lot more information on this in the man pages so take a look there also. If you dont know your average system load you will not be able to determine if the load is in a peaking state or if the host is able to handle more load.
Note that some commands might require installation of a few packages.

Below gives a nice simple performance view of a HDU. Ofcourse running...