If they are using something like a USB modem or you just want to test the connection from the client, then I would imagine that the easiest way to monitor would be to use some sort of monitoring utility on that PC. If I recall Analogx provide some free tools for such a job.

You also need to define a strategy since a problem might exist with the client PC, ADSL device, cabling, telephone exchange or just simply conditions on the internet at that current time.