How do I indentify fabrics?

I didn't want to make my first post a question but I didn't see an introduction section  So here I go.

My sister is having a baby & has asked me to make her a few dozen flat folds so she can cloth diaper. My aunt taught me to sew on a machine almost 8yrs ago but the moved away soon after, so I'm still pretty clueless about it. I have the machine manual & I can run it like a pro but I just sort of played with it on & off. Learning but not really learning correctly. Just playing, so I still don't know ALOT, I've just been kind of winging it 
I also had a neighbor donate me a MASSIVE amount of fabrics, she used to quilt/sew alot but she's getting older now.

Soooo, how can I tell which fabric is which without having to ask her & look like a fool? I tried the burn test, apparently it's not an idiot proof test because I still can't figure it out! I can post pics if anyone can help me.

Thanks in advance to anyone who can help!!

Cotton or hemp will be the most absorbent for flatfolds. Especially anything with a looser weave or a knit. I know many people use old tee shirts. The good prefolds are made from birdseye weave or twill weave. These can be purchased fairly inexpensively by joining a co-op on yahoo groups. Sorry I can't recommend a specific one right now. You might check diaperswappers.com. I think they also have a DIY forum.

If you aren't confident about your burn test results you can also try cutting a strip of fabric, pour water on one end and see how fast it wicks toward the other end. If it beads up you obviously don't want to use that fabric as a diaper soaking material! But if it soaks up fast (think old paper towel commercials) then it will be a good diaper material.

Another way to learn about fabric content is to go to your local fabric store and feel different fabrics and check the contents. Blends are tricky. But if you compare a known fabric to stuff you have at home it may help you learn to distinguish. The weave and finish on a fabric can be deceptive. It has taken me years of sewing to be able to guess at fabric content and I can still be fooled. But it is a skill worth working on. And try the burn test with a known fabric so you can observe whatever written descriptions you are reading.

Best wishes, I hope your sister reallly appreciates you!

Thank you random-charm  I will defiantly be using those tips! I have some old tshirts with logos on them (Fruit Loops, prison guard, Mexico, etc) that I plan to use too because their just so cute. I'll post some pics when I get them done.