If you are using cgi, just time how long it takes to send some # of bytes to the user. I would think that this is going to take some tuning, as it is very hard to tell a bad connection from a slow connection.

You need to think about how short term congestion on the network will affect what you think a users bandwith is.

You might want to look at rrdtools http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/. It would let you keep a history of bandwidth and get averages over some period of time.