Depending on your network card and Operating System you may be able to guage the traffic you are using on your own. If you're using Windows 2000 or Windows XP Pro/Home you can right click on My Network Places and select properties, open the local area connection that is connected to the internet. There you will see a section listed as activity. If you're lucky your network card will list it as bytes instead of packets. Now this only works if 1. you're connected to the internet via a broadband connection. 2. that broad band connection is connected to your computer via a Network adapter. 3. You haven't be doing a lot of surfing of your local network as the activity lists all the traffic the network card handled including the request for an IP address.

If it lists packets instead of bytes then it's a crap shoot because a packet can be any size as the packet size you receive is predicated by the sender and any router/system in between. You could multiply those packets by 4KB which is a common packet size but that's not always the case. The only other way that i am aware of is via logs on a router your connected to (Linksys, D-Link, Netgear and others sometimes keep logs) or with a network analyzer some of which cost big money. You may just be better off selecting a middle of the road package and keep an eye on your usage. If it's high then cut back and if it's low then ask for a lower package the next month.