You use Network Utility for that which can be found inside the Application/Utilities folder. ~hacksaw
ping -c 100 google.com (type this on terminal app) this pings the site google.com 100x and gives you the results after, just change google.com to any site of your choice. HTH
Like guwapito said, the Terminal is all you need to determine your latency to specific sites and servers.
This terminal method guapito and cake mentioned - is the method I use also because it is FAST and no need to load pages or anything. Just type and its done. But thanks guapito for showing me the extra -c 100 command. I had been for years just manually doing it several times without the -c 100. That extra 'switch' makes it MUCH NICER !
