To make a rough test of throughput, simply create a huge object and transfer it through an ObjectOutputStream and time it with System.nanotime(). Or, generate a large text file and use a reader to push that through a stream. However, there are myramid reasons your results could be off, such as Comcast's powerboost or whatever they call it.
Any particular reason you want a bandwidth test? Professionals have done it a lot better, and their results will be an order of magnitude more accurate and repeatable.