Q. Can a shaatnez tester in case of need use a strong magnifying glass or compounded instrument to check for shaatnez during Shabbat?
A. Shulchan Aruch (O.H. 307: 17) writes that the ones that permit the studying of sciences during Shabbos permit also the use of an ‘astrolob” on that day. Mishna Berura (ibid.66) explains that it is a telescope to observe the stars. Based on the above Poskim permit the use of binoculars or a telescope, provided that there is no other prohibitions involved such as carrying in a public domain, use of electricity, and on the condition that special skills are not required to assemble the equipment. They also permit to adjust binoculars and a telescope in the normal manner. (Shmiras Shabbos Kehilchoso 16: 45. However, on note 111, he quotes an opinion that a telescope may be muktzah machmas chesron kis.
Shulchan Shloimo (308: 22) debates if the use of a microscope is similar. However the use of a hand held magnifying glass even if compounded by the use of several lenses, seems to be permitted.
It would be prohibited to dismember or take apart the clothing material to observe the nature of the threads. But if just by examining with the instrument the material as is, one could tell whether it is shatnez or not seems, to be permitted. It is possible that if the clothing needs testing, it could be muktza. (See Hashatnez Behalacha 10: 47)
Horav Shlomo Miller’s Shlit’a opinion is that as long as the test has not proven that it is shatnez the clothing is not muktza and it can be tested with magnifying glasses.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a