It depends which game you are playing when checking your ping. By default many games allow you to check scores and ping at the same time by pressing TAB. Others use Tilde (~) or F1.
Jedi Knight used Tilde (~), Quake 3 used TAB and Elite Force by Raven used TAB as well. The good news is that you can set whatever key you wish to display pings and scores when you configure your controls.
Also, this thread is being moved to our off-topic forum here
as this forum is only used for dicussion of JK2.