If you have a system that you can test against (i.e. a server at your work with a fatter-pipe then you have at home, or a hosted server/VPS/etc.)
iperf
run "iperf -s" on the server and "iperf -c server.ip.address" on the client.
Read the man pages for more options.
If you don't have a 'known better then you' to test against try this to test your maximum download bandwidth.
Simple test: download a large file from Microsoft (i.e. a 'network install' service pack, or similar) or other big-host
More complicated:
run several (4-20) 'wget' concurrently. If you use Linux .iso's as your target download, make sure you grab the files from *.edu sites. Schools should have a lot more bandwidth then the average .com that is hosting files.
Your ISP might have several things in place from preventing DDOS attacks from there customer machines. So each 'download' might be throttled by your ISP. If you open several download threads to different locations, downloading different things you can maximize your usage.
Also, don't download the same thing twice from the same source. Caching can/will interfere with accurate measurements.