Hey Furrball,
Good question!
There isn't a way of monitoring your usage with the Top up and Go Broadband dongles at the moment, apart from the amount of credit you have going down.
It's still a fairly new product for us at the moment so there's a chance that something along these lines could be introduced in the future, so keep an eye on the forum/website for more information
In the meantime, it's worth checking that things like Windows Updates, Virus Checker updates and basically anything else that's running in the background are either stopped or up to date - as these can take up a fair amount of data depending on how big the respective update is.
If you've got any other queries then let us know, otherwise I'll see you around the forum!
All the best,
MatC
eForum Team