How do I know if something really has been 'ergonomically designed'?

Try using it. Think of all of the ways and circumstances in which you might use it. Does it fit your body size or could it be better? Can you see and hear all you need to see and hear? Is it hard to make it go wrong? Is it comfortable to use all the time (or only to start with)? Is it easy and convenient to use (or could it be improved)? Is it easy to learn to use? Are the instructions clear? Is it easy to clean and maintain? If the answer to all of these questions is 'yes' then the product has probably been thought about with the user in mind.