If you have a static IP address, you can use the TBB service here:
http://www.thinkbroadband.com/ping
If not, you can use something like No IP (
http://www.noip.com/) and f8lure:
http://fruk.net/index.php?fruk=f8lure
The general gist of these services is that they constantly send pings (tiny, tiny amounts of data) to your broadband connection and measure the response time, and count how many (if any) pings get lost / don't get a reply.
The tests effectively check your connection 86,000 times a day (once per second) and give you a very clear idea of how well your connection is performing. Setting up the test won't have any impact on your connection - although one test a second sounds a lot, the amount of data used is negligible.
eg
My Broadband Ping