ובגד כלאים שעטנז לא יעלה עליך
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)
Therefore 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, that is considered
Hana'as Levisha, if you leave the dressing room to
a public area.(mirrors etc.)
Therefore you may not leave the dressing room while
wearing these types of Shatnez garments.