This is a pretty naive question, I guess. I frequently need to figure out what if the problem with a web app or is it simply a networking issue – lost packets or a slow connection in any point between, say, your boss on a crappy wifi connection at the airport and the web server, or the CDN and the web server or MySQL and the webserver, or memcached box… You get the idea. Also, how do you tell if it's the DNS server?
Some of you are missing the point. This is not a question about measuring web application performance. This is a question about measuring network performance as a component of web application performance.
Best Answer
Set up tracing (perhaps togglable by a querystring), showing the amount of time spent in heavy sections of the code, and overall server time. If it server creates the page in .2 seconds and it takes 10 seconds to load, you know it's either the network, or the user's browser.
You can also use the Net tab in Firebug, or timeline view in Fiddler to see individual components coming down and how long they take.