A polarizing microscope is a quick method to identify textiles such as painting canvases. To identify common historical textile such as cotton, hemp, wool, silk and flax you have to look at certain features. For example, cotton is easily recognized because its fibers show a characteristic twisted shape while hemp, jute and flax are pretty straight. Flax and jute are recognizable because of their nodes. Jute also has tapered ends.
References about polarizing microscopy, preparation of slides and textile microscopic examination are
This is a video made to illustrate some fibers’ features as seen under a polarizing microscope.
This is my flowchart for preparing a textile identification report. W. C McCrone (1982) “The Microscopical Identification of Artists’ Pigments” Journal of the International Institute for Conservation—Canadian Group 7 (1–2):11–34.  N. Petraco, T. Kubic (2003) “Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators”  B. Wheeler, L.J.Wilson (2008) “Practical Forensic microscopy”  W. C. McCrone, L. B. McCrone, J. G. Delly (1978) “Polarized Light Microscopy”  K. Vanderlip Carbonnel (1980) “A study of French painting canvases” Journal of the American Institute for Conservation, volume 20, number 1, article 1 (pp 03-20).  R. D. Harley (1987) “Artists’ Prepared Canvases from Winsor & Newton 1928-1951” Studies in Conservation, Vol. 32, No. 2 (May), pp. 77-85.