Q. Do you have to check for shaatnez a suit that according to its label it 100% polyester etc. and does not contain any wool?
A. Many of the shatnez cases in our days are likely only Rabbinically prohibited. Shatnez is the acronym for “shua, tovi and nuz – combed, spun and woven,” which describes the stages in processing fabric: combing the raw fiber, spinning fibers into a thread, and weaving the threads into cloth. For the prohibition to be Biblical, according to Rashi (Yebamos 5b, Nidah 61b) and other Rishonim, the wool and the linen, would have to undergo the three stages mentioned above. Tosafos maintains that wool or linen thread combed, spun and twisted separately and then connected together will comprise shatnez Mideoraisa. There are different opinions as what Rambam maintains. Tur opines he follows Rashi, others assert he adds that any combination of wool and linen connected together, including felt – which is pressed wool and linen – is considered Biblical shatnez. (See Shulchan Aruch Y.D. 300: 1)
Horav Shlomo Miller’s Shlit’a opinion is that since in principle the prohibition is likely only rabbinical, if a sofek or doubt exist if there is any wool at all, it would be permitted. Therefore, if the label reads that the clothing contains no wool, it should be in principle permitted.
However, when possible one should consult with the experts in shatnez testing if they have any knowledge regarding this specific clothes manufacturer and the reliability of his label.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a