Client to Server ping: This one's pretty easy. Simply timestamp your packets and find the difference between the times when the packets arrive. Take the average of these times over some period (say 1 second) and that's your lag from Client to Server. As far as display, that's totally up to you and your game layout. In general, it's a good idea to display lag as part of the "score breakdown" screen, where each player's public information can be accessed. Also, you may consider color-coding lag levels; such as green for <100ms, yellow for <200ms, and red for 200ms+.
Client to Client ping: I'm not sure this is implemented very often, and probably for good reason: it's unimportant. Actually, ideally, there is no Client to Client ping because there is no Client to Client interaction; all interaction takes place on the Server. If you did have Clients connecting to each other, you would probably display that information in much the same way you would as a Client to Server connection. However, if you wanted to display ALL of the pings between ALL of the Clients, a table would work.
Hope that helps!