battle test your server with siege
“Siege is a multi-threaded http load testing and benchmarking utility. It was designed to let web developers measure the performance of their code under duress. It allows one to hit a web server with a configurable number of concurrent simulated users. … Performance measures include elapsed time, total data transferred, server response time, its transaction rate, its throughput, its concurrency and the number of times it returned OK.” (source)
Here is a gist that shows how to use it.
siege only on your sites since it can be interpreted as a DDOS attack.