I'm not sure if this works on Verizon or Sprint, it should, but it definitely works on AT&T:
Go to Settings > Phone > (AT&T/Verizon/Sprint) Services > View my data and Msg
It'll show you how much you've used and how much you have left.
I used to have the limited 200 Mb data plan on my iPhone, so what I did is I disabled all push notifications, because that uses internet. I also didn't watch any YouTube videos or used music streaming apps like Pandora or Spotify. I just browsed and used Facebook, and I had some left over. I could watch a Youtube video or two, and I would be fine. Also, don't download any apps.
Feel free to do all of the things you love about the iPhone while you're on Wi Fi, but just be aware of when you're not.