Network problems are in many cases quite easy to detect if you have a closer look at the response times. You will see some unusual spikes in the response times which randomly switch from low to high, and additionally, that many of the spikes have almost the same value. The following picture shows a real measured network problem. The most spikes are at 17,000 milliseconds and some few of them are at 40,000 milliseconds response time.
Note that an overloaded Web application never does switch form slow to fast response times (and back) under same load conditions. Therefore if you see such an image this is not a problem of the Web application software.