How can I tell how much of an internet pipe is being utilized?

We currently have a 10mg internet pipe in our home office. We have several small remote sites that have local,(DSL, Cable, etc) connection. We will be moving to some new software that is web based and our remote sites will not longer access via Citrix from their local connection but will be tapping into our 10mg pipe, since the new software is Web based. How can I tell how much of the 10mg is currently being utilized, so I can determine if/how much I need to increase that 10mg by?