Let me share EXACTLY HOW I KNOW. I log onto my sprint.com web page and on that is my usage numbers.
There are apps for PCs such as NETLIMITER NETMETER and such but I find I had to look at my provider's numbers all the time.
My 2 room mates and I have a USB broad band modem: "Novatel Wireless Ovation? MC760- EVDO Rev A", which we stick into a 3G router "Zoom 3G Wireless-N Router- Model 4501, Series 1075." We have a 10GB/month data plan, but no idea of how to track how much of this allowance we are using. Does anyone have any suggestions on how to track how much we are using so we can do things like watch videos online without going over our limit and paying extra fees? We'd need something that either sees what the card is doing, sees what the router is doing, or tracks what each PC uses ONLY on the home network and then adds it up online or something. N-telos, whom we have the router from has no idea how to do this. Does anyone else?