According to 9to5Mac

Beginning this week, when a customer is interested in purchasing in-ear headphones, an Apple Store employee will have the ability to create a custom Demo Kit that holds up to six different models of in-ear headphones. These kits will include a fresh pair ear tips so that the try on process is sanitary. The customer will be able to keep the ear tips regardless if they purchase the headphones. The customer can then plug the headphones into their iPhone or iPod to sample the quality before making their purchase decision.

Some of the headphones that reportedly will be available for testing in Apple Stores are the RHA MA450i, the urBeats, the Beats Tour, the JayBird BlueBuds X, the PowerBeats 2 Wireless and the Bose QC20.

Source: 9to5Mac