Answer:
   
   `Shaatnez' is the occurence of wool and linen in the same garment.
   There are various prohibitions (Lev 19:19, Deut 22:9-11) against the
   mingling of different kinds; this is one of them. A linen tie worn
   with a wool suit is permitted, but a wool suit with linen threaded
   buttons is prohibited.
   
   While in practice, many garments do not have any Shaatnez and may be
   assumed to have none, the particulars vary by garment type. The
   padding in many garments such as suits or the embroidery thread, such
   as designs on sweaters (men's and women's) may cause shaatnez
   problems. The padding filler in many suits is made of assorted rags
   which may be mixed linen and wool in themselves (so it is not just a
   worry of linen threaded padding in a wool shell suit).
   
   Nowadays, the usual way of observing the Shaatnez prohibitions is to
   first check the fabric list (careful: lana/lino is Spanish for
   wool/linen). If the fabric list shows a forbidden mixture, don't
   bother, you probably can't get it fixed. If the label shows "other" it
   may or may not be linen. Even if the label shows 100% wool, there may
   still be problems.
   
   Since the fabric list on suits usually refer only to the shell (and
   ignore padding or ornamental threads), the label can only be used to
   identify garments that definitely have shaatnez. Thus if the label
   indicates that the suit (for example) can be good, take it to a
   Shaatnez lab for testing. Most cities with at least a medium sized
   Orthodox community have qualified Shaatnez testers. If the city has a
   local Vaad Hakashrus they can usually refer you to a reliable tester.