How do I prevent excess data usage?

The last two months I have gone over my data limit on my cellphone plan. I am constantly connected to WIFI at home and at work I only use facebook on my phone. I understand with the WIFI dropping out throughout the night might cause my phone to use the 3G, however before I got WIFI I never went over my limit and my usage hasnt increased. I hardly use my phone for anything except facebook and snap chat and it has never been a problem before. I have a feeling my phone is using data in the background somewhere, I have closed all my apps and I havent updated anything. Apart from switching my data off entirely is there anyway to prevent this happening or to see what exactly is using all the data?