ובגד כלאים שעטנז לא יעלה עליך

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.