Late to the party, but this is what I was taught when still new to things and it has saved false economies.
If you are not sure of your fabric content, you need to take a sliver of the fabric in question and a lighter - this is best done over a sink with some water in it - and do burn test.
Cotton and linen will burn clean and leave a light ash, if anything.
Wool will smoulder before it burns and silk will burn quickly. Both stink of burnt hair and leave a crumbly dry ash.
Anything with 'esters in it (ie plastic additives to either bulk out the content or to add durability) will burn with more or less black smoke, and form a black, bubbling ball, which, when cool, is hard. Do not touch it when hot, as it sticks and makes a prolonged skin burn , on the napalm basis.
Wool mixes, depending on the amount of polyester added, will stink of hair and also bubble a little.
Polyesters etc can mimic many natural fabrics very quite convincingly nowadays - it is after all only a filament, the spinning and weaving of such determines the end appearance and handle. Hence using only a sliver of fabric, over a sink with a little water, in case your fabric surprises you and burns hard and quick.
Fuller details here.
http://www.threadsmagazine.com/item/314 ... -burn-test
And you buy wool for more than authenticity - ie thermal reasons - one good layer of unlined, combed or napped wool is much warmer and more efficient than a lighter plain weave or a wool-alike even if lined. It is also going to repel water for longer (and can be treated with lanolin substitutes to do so.) From a point of view fo cost, investing in good cloth pays for itself, as they double as blankets, besides and later providing much uncluttered yardage for recutting into other garments.