See the offloading percentage of a varnish installation?

[Redalert Commander]

Hi,

I'm a system engineer with limited knowledge about varnish, I do know
it is used for caching.
One of our clients uses varnish on their website, but I'd like to know
the offload percentage.
How much traffic reaches the Apache server behind it, and how much is
handled only by Varnish's cache(s)? Is there a relative simple way to
detect/query this information?

We run varnish 3.0.x.

Regards,
Steven