One tactic is to substitute your competitor’s name in place of yours in the unique selling proposition that you have created. Now show the unique selling proposition to employees, customers, prospects, analysts and influencers. Does the unique selling proposition still have credibility? If so, start over because you have not identified the uniqueness / differentiation.
Another way to conduct the test is to document the unique selling proposition but leave the name of the company out. Show the value proposition to employees, customers, prospects, analysts and influencers and ask them to fill in the blank. This approach tests for unaided awareness. Once you offer vendor names the test turns to an aided test. In the later scenario, introduce one vendor name at a time and ask if the statement is believable. Test all vendor names in the same manner (pronunciation, same amount of time allowed to respond, same person introducing the vendor names, etc.) to limit bias.
You can read a relevant blog post on the topic at Developing a unique selling proposition.
There is also a good template that walks through the process of market landscape, competition and differentiation that will facilitate the creation of a unique selling proposition.