How do you know if you are getting value from your PR agency?

Before outsourcing you need metrics in place.
What are your expectations and what do you expect the partnered PR agency to do for you? Is it to increase your visibility in the market by a certain percentage? If so what tools is the company going to use to measure that? Or maybe you measure perceived value by an increase in patronage? Whatever the case set those metrics and create a time schedule.
Hold the company to those metrics.
For example; you want to see a increase in your patronage by3% in6 months.
Before going into partnership with this company ask them if they can meet your goals and time parameters.
If they say yest the can then hold them to that.
if they don't meet your requirements then you are not getting your value.

If you see creative and logical Advertisements , Varieties in the products , Real tangible scale for supply and demand , those are indicators for what you asking about ...

BY CHANGING MY THOUGHTS AND BEHAVIORS

The effectivness of tyhe agncy can be determined  from its working reationship truck record  or from its wide experinece recordes it has accomplished over years.

When companies sign  PR company, PR objectives are set and deliverables are agreed. The progress of the agency work is monitored based on the same deliverables and KPIs. If the brand is achieveing its objectives than the progress is good ... also if the goodwill of the brand has increased it is also says the same thing.