Suppose someone is fabulously wealthy, and they have their own personal tailor who is not Jewish. May this tailor be relied upon not to make anything which contains Sha'atnez? May this tailor be relied upon to check for Sha'atnez in his materials or in whole garments purchased elsewhere?
Per Yalkut Yosef when you purchase clothing from a non Jew you have to inspect them for Shaatnez even if the non Jew insists there is no linen threads. However if linen threads are significantly more expensive then one may rely on that fact. If one supplied the material to a non Jew we are not afraid that he will switch the goods, since it can be checked.