I would suggest you listen to the askhistorians podcast, episodes 45 and 46 . In this u/chocolatepot and u/kittydentures talk about some areas of study for fashion history and the methods involved. It starts mostly with paintings and later you also get fashion magezines if I recall corectly but give these episodes a try because it’s really fascinating.

It can be difficult! While there are scholars with all different specific specialties, the field in general takes a kind of top-down approach - that is, the study of the most fashionable dress tends to predominate. We use the more elite dress to create a framework for understanding clothing in general, and then, typically, branch out into specifics.

From the late eighteenth century forward, fashion plates are a wonderful source of information. Because the entire point of them is to convey information about dress, it's not surprising that they show cut, color, accessory, etc. in great detail! They're also very often associated with specific dates that can be relatively easy to find, since they were published in magazines, and we can presume a certain accuracy due to their commercial/educative nature and the incentive to depict up-to-date fashions. If the fashion plates published in late 1892 show extremely full sleeves on women's gowns, then we can say that "leg o'mutton" sleeves were being worn at least that early.

Now, the flip side, relevant to your question, is that those fashion plates do not prove that nobody was wearing narrower sleeves in late 1892. The most elite would purchase high-end gowns direct from Paris, London, and/or New York (many of which are still in existence in similarly elite institutions like the Victoria & Albert Museum, or the Metropolitan Museum of Art), and the upper middle class would follow fashion plates avidly, but lower down the social scale, women could simply not afford to be so au courant. We have to figure out what was considered laughably unfashionable to everyone, what was a "normal" level of fashionability/unfashionability, and what resources the poor and lower middle classes had.

This information is not generally conveyed with the same level of detail in cut and construction that we have when it comes to fashionable dress. Records of what English parishes gave to the poor or were asked for, for instance, often give us numbers and perhaps basic materials, but don't explain the styles items were made in: we can see that shoes made up the largest proportion of donations, with body linens (shirts and shifts) next or farther down the list. At St. Martin's, Westminster, between 1800 and 1818, 1440 pairs of shoes were given out, 1132 shifts, 520 aprons, and 231 petticoats: women's shoes were most commonly wool and men's leather, while shifts and aprons were typically linen, but we can't say what the petticoats were made of. In the St. Martin's records, there are no stays/corsets, suggesting that either the very poor women performing manual labor didn't wear them, as they simply had to focus on the even more basic body covering and foot protection, or that they were expected to make their own repairs on these valuable and difficult-to-make garments. In some cases we also have the opposite records - what fabric was bought by parish authorities. These don't show us what exactly was made, but we can see lots of cheap calico (printed cotton) and wrappering being purchased, implying that many of the garments coming out of the workshops were made of them. Similarly, there are wills that record amounts and types of garments, sometimes with a basic description of fabric, color, and quality.

Another very useful source that takes a third angle on the matter is fiction. Descriptions in novels and short stories obviously do not give hard statistics and are not based on concrete examples, but they can often show what is seen as normal and expected, particularly if the novels were seen as realistic and authentic in their time. [Insert disclaimer about the meaning of "authenticity" and its differentiation from "accuracy" here.] Charles Dickens and other authors (plus their illustrators) showed a great range of clothing worn by laboring characters, representing working people from the "shabby genteel" who worked hard to present a middle-class appearance to the class who worked in service or as artisans to the truly destitute. Generally, this poverty was described as having slightly shorter garments (necessary to keep skirts clear of feet or trousers out of the dirt), fewer effective outer layers or undergarments (worn out or pawned), less formal headgear, worn-out shoes, threadbare material, dirt and dust on fabric, secondhand or thirdhand garments, patching, and literal rags or nakedness at the very bottom of the social scale. There are also formal liveries worn by servants, and traditional but not official "uniforms" of various professions, such as the smocks worn by farmers.

We also do have extant "everyday"/"ordinary" garments made of cotton, showing wear, and with very little ornamentation. It's the fanciest garments at the biggest institutions that get the most attention in exhibitions, because they're very appealing visually, but collections actually tend to consist of a great range of clothing. Individuals did tend to save their best - so the really workaday pieces are rare - but individuals existed at all points on the class spectrum. Unless an extant garment is associated with a particular occasion, such as a named person's wedding, it can be impossible to date with the certainty that a Worth or Doucet gown that resembles a fashion plate has, but we can usually see a continuity of sewing techniques and basic construction: the main differences are often in the cost of the fabric and the lack of trim or decorative cuts.

A couple of good references on this:

"Reclothing the English Poor, 1750-1840", Steven King in Textile History (2002)

"'The Burial-Place of the Fashions': the Representation of the Dress of the Poor in Illustrated Serial Prose by Dickens and Hardy", C.M. Jackson-Houlston in Textile History (2002)

"'Cast-off Wearing Apparell': The Consumption and Distribution of Second-Hand Clothing in Northern England During the Long Eighteenth Century", Miles Lambert in Textile History (2004)

"Clothing the Poor in Early-Nineteenth-Century England", Peter Jones in Textile History (2006)