Is it possible to view data usage

Could anyone kindly inform me if it is possible to view what data has been used over a one month period.

I currently have a SIM card in a router which provides my TV, PC & Tablets and phone Wi-Fi signal. The SIM deal is for unlimited data. I suspect with only two of us in the house unlimited data could be trimmed down to a cheaper option but without knowing what data we use per month its difficult to choose another cheaper deal.

Any advice where I could view our data usage would be appreciated.