How can one try on clothing before checking, if it might contain Shatnez?
The Mishna in Kelayim Perek 9 says that those that sell Shatnez are allowed to try it on in order to model it to the Purchaser. According to this Shita you would be allowed to try it on prior to purchasing. However we do not Pasken like that. The Heter to try it on is based on Rov, since most clothing is not Shatnez, and as it is not yours you can not check it. However once you purchase it and you can verify whether it is or isn't Shatnez then you are not allowed to wear it till it is checked as it is (Efsher Livarer) possible to check.
Heard from one of the checkers at the Flatbush Shaatnez Lab