Jacob Stainer was one of the most famous early violin-makers, and the first well-known one from what is today Austria. His violin designs became highly influential among makers worldwide, and his instruments were the most sought-after in Europe up until the end of the 18th century. Today, his violins are rare, and remain highly desirable to musicians who play early classical music. It is not known exactly how many exist. Identifying Stainer violins is difficult. Stainer never made any two violins exactly alike and, because of their popularity, his violins were the first to be copied, complete with counterfeit labels. (Reference 1)
Look at the shape of the body. Stainer violins consistently have a relatively broad lower back and a pronounced arch in the belly relative to the back. When held horizontally you should be able to see through both F-holes. The corners of the middle bouts, or wider sections of the instrument's hourglass shape, extend little compared to other violins.
Look at the craftsmanship. Stainer is known for his highly skilled craftsmanship. Many of his violins are adorned with cut scrolls, and sometimes with carved lion's heads, angels or women. The F-holes have circular ends. They are small, and sometimes asymmetrical, with the right one higher. (References 2,3)
Look at the colour. The varnish, similar to varnishes from the Cremona area of Italy, vary from totally transparent to yellow, brown, orange to chestnut. Some have a light yellow belly and are dark brown everywhere else. (References 2,3)
Look for a label. Stainer hand-wrote the labels on his violins. A typed label indicates that the violin is probably not a true Stainer, or that a label was made later by a dealer and attached. (References 1,3)
Given the rarity and value of Stainer violins, this should serve only as a rough identification guide based on the characteristics of Stainer violins. It is highly advisable that you consult a violin expert for any true authentication rather than try to do it yourself.