Assuming (with a tip of the hat to misterbwong) that you want to demonstrate the impact is real by measuring it, A/B testing would be one of the more popular ways to do it nowadays. The kinds of questions you can ask with it are somewhat limited, so you've got to be worried about the trap of searching where the light is better. But it is so very cheap and easy.
If you have more resources and need to ask questions A/B testing doesn't handle well, you can also do it with good old-fashioned surveying. Take (say) a few hundred people sampled as representatively as possible from your target market. Randomly show half of them a version of the design that uses one typeface, and the other half a version that uses another. Ask them to rate how well they liked the design. See if there's a difference. Or ask them to recall things they saw, and see if there's a difference.
