Verizon Wireless has a neat feature :

dial *MIN and it tells you how many peak/offpeak minutes you've used in the current billing cycle.
The call is completely free so call all you want.

I also called Customer service and changed my billing cycle date to the 1st of the month.
that way there I can *MIN and find out how many minutes I've used, and figure out how many minute I have until the end of the month.