We’re currently running our docs through usability testing too. We’re using a company called “whatusersdo”, not free I’m afraid. However, it’s been very helpful so far…when they read the instructions you give them that is!
We’ve also done some internal training too - making sure those who actually use the docs daily get to play with it, using the same kind of tests.
That way we’ve had a good balance of those who are purely there for testing its functionality, against those who know the ins&outs and so who might have a bit of industry insight and will already have an opinion of what they need from them.
Hope that helps in some way!