No known way to check as far as what Verizon offers. You can "guesstimate" based on some of the counters indicated in the modem's Statistics pages but by no means will it be accurate. If you use a third party router or have third party firmware installed to a router, there's a good chance it may have some sort of way to monitor your data usage. My DD-WRT powered router for example does, and I'm using a couple hundred Gigabytes a month.
Either way, if it's just because you're curious then that's fine, but otherwise I wouldn't worry about it. Verizon is keeping their DSL product unmetered as of right now, and rightfully so. I can't say much about Verizon Wireless's HomeFusion service though where they HAVE data caps and must supply a way to monitor it.
It's funny, but I just asked the same question elsewhere [the search function didn't return this thread].
While I know that DSL bandwidth is "unlimited" from a customer standpoint, I also know that a *lot* of work goes into setting things up such that "excess bandwidth" isn't sitting out there unused/unsold.
There has got to be some way to determine, even if it's roughly, how much DSL bandwidth one is consuming. The question is, how?
For those of us thinking of going truly wireless, and with most MiFi plans not offering unlimited usage, it's almost critical to know what your existing DSL bandwidth usage is before choosing a starter plan.
There are some other, per-PC solutions that you can use that monitors bandwidth on a particular computer, but they are easily skewed by traffic within your home network and don't quite work completely on mobile phones, that require their own apps to measure data usage.
If anything, you may be able to get some logs from Verizon if you can get the right person to take a look but I doubt they'll cough up much given how they currently manage bandwidth usage (aka, folks on FiOS doing 30TB/m without hearing a peep from Verizon).
I can only suggest using a third party router with built-in bandwidth monitoring and connecting your entire home network up to it. for a month or two. That should give a good gauge of how much you're using. If you Netflix or have a home of gamers, expect a Netflix movie to use around 1GB a movie, expect games to use a small amount of data around 300MB a month factoring in small patches and online play, and for any additional traffic such as YouTube, give or take about 30MB per video implying some length, or some viewing of short 720p content.
If you're multimedia or video game heavy, a capped plan won't be for you nor would Wireless. But, I won't be the judge