How do test/compare an existing function with prototypes

I want to compare an existing function in an existing solution (search filters not being seen and/or used on our corporate intranet) with one or two prototypes.

What's the best way to this? How do I compare and measure and verify if something is better/clearer easier to find without telling the test person what I'll be testing? The problem, or problems, are:

  1. They know that they are being tested.
  2. I'm comparing a fully working existing solution with limited prototypes (thus the scope is limited) and hence then know they are tested.
  3. And because of this limitation, they know what I'm testing. So, if they normally don't use the search filters (either because they have never seen them or knew how to use them) they will for sure try to figure it out now in order to complete the task.

So, how would you go about this?