There are so many things to know about fabric, and so many ways to know it, the least of which is by resources online. I say that with no disrespect to all the truly helpful online fabric resources making an effort to share information about distinguishing fabrics.
It's just that fabric is about half appearance and half feel, and even the look is different in person than when on screen.
Before I get too far: I really have to know fabric better all the time. I sell vintage clothing, and my buyers and I want to know what a thing is made from. To know this is to tell someone whether she will be allergic, how to wash or clean the item, predict dye-ability. It is to know how fine it is, how long it will last, how the color will hold up. It helps make certain the vintage. It gives a better sense of how it will feel when worn. Buying clothing online is hard enough, and knowing all you can about the item is just smart.
Let me start with one colossal resource in the form of a heavy-duty dictionary, Fairchild's Dictionary of Textiles 7th Edition, Phyllis G. Tortora, editor, Robert S. Merkel, consulting editor, Fairchild Publications New York, 1996.
This thick volume, which runs about $47.00 new and not much less used (there are previous editions, which I've not seen, for less than half used) is a necessary compendium. There is so much more to each textile defined than expected, and often I get stuck reading and reading. Today it was muslin, and here is the whole definition to give you an idea of the depth of this book:
"A large group of firm, plain weave cotton and cotton blend fabrics in a wide range of qualities and weights from lightweight sheers to heavyweight sheetings. May be given a great variety of finishes. Muslins are used for many purposes, such as underwear, aprons, linings, shirtings, dress fabrics, sheets, pillow cases, furniture coverings. Muslin is one of the oldest staple cotton cloths and was first made in Mosul, Mesopotamia (now Iraq), where it derived its name. According to Marco Polo, at the end of the 13th century, fabrics made of gold and silver thread in Mosul were called mosolin. During the Middle Ages, applied to heavy, coarse cotton fabrics made in Mosul. India then began to produce a variety of fine cotton muslins, often printed with gold and silver leaf. For a long period, muslins were imported by European countries, especially France, from India. They first were made in Europe in Paisley, Scotland, about 1700. SEE SILK MUSLIN."
The Fairchild's is a fabulous resource for history, usage, type of weave and fiber, commercial treatments, and also business and trade names. Maybe you can guess the problem: If you don't know what muslin is by look and feel, how will this help? The Fairchild's has photos here and there, but they are small, and many fabrics are pretty similar in appearance when seen in a small black and white photo. Fortunately for someone like me, there are ways to supplement this type of resource.
Next time: Swatch books, your own or commercially-made.