According to Mori V'Rabi Hagaon R. Shlomo Miller Shlita,
when trying on Shatnez garments , you may do so
only if you don't have Hana'as Levisha (benefit in wearing)
You may try it on, for size or looks.
When trying on, pants skirts etc. being it's embarrassing
to walk around in public without them .It is considered Hana'as
Levisha.if you leave the dressing room.(public area)
Therefore you may not leave the dressing room while wearing
these types of Shatnez garments..