Open a command prompt on a client PC, via the Start Menu search for "cmd".
Type "ping -n 100 google.com
This will ping google.com 100 times. Substitute google.com with whatever address must be tested to.
Once the command has run, a summary will be presented indicating if loss occurred. This will test for ms response time, and at the end of the test it will tell you if there is any packet loss. Run this test a couple times try different addresses. Run is when you know you are experiencing lag. Don't run is while playing a game. Exit the game. Let me know what your results are.