Can I use this fabric?

Thriftiness and cloth diapering often (but not always) go hand in hand.  Consequently it is common to hear "can I use this fabric" with a picture or vague classification.  I understand and applaud the thrifty, always looking for a deal or upcycle attitude.  I do.  But, this is going on your baby's bum, right next to their skin, in the region of their genitals.  For this reason mystery fabric is out in my opinion.  If the fabric and fiber content is clearly labelled or easily identified check the fabric reference page here.   You can also fine it on the right hand side of the blog with a list of key pages. 

I found Fleece
Fleece blankets often show up for sale in stores.  If its cotton,  hemp or bamboo-rayon it is absorbing.  But most fleece in stores is poly.  Issue here is which poly fleece?  You can water test to see if it beads (water resistant), or if it wicks (brings water through but feels dry-ish to the touch (micro fleece, usually very thin).  The problem is you have to wash fleece first to see how it will preform.  I've tested fleece blankets at home and not found one that was ideal for making an HF.  You can test it OTB (on the bum).  But be prepared that it may not preform. 

Bathmats, cleaning rags, sham wows, and other fabrics not designed to be worn
Key here; not designed to be worn. 
Yes you could possibly use it.  However it is going on your baby's bum and it wasn't designed too.   I recommend passing on these items.  It gives me the heebie geebies thinking of diapers made of such items.  Who knows what's in it? 

Shower curtains and other water proof items:
No.  Just don't.  Yes it would be waterproof as a diaper cover, aio or pocket.  But still don't, it's a great way to end up with baby having diaper rash and or heat rash because it's not breathable at all.  Yes someone came up with the first plastic pant this way, it became popular and was used for a few decades by some.  But during those decades there was a resurgence in diaper rash that hadn't been seen since the dirty dark ages of Europe.  We have pul and tpu and water resistant fleece now, we've always had wool. Make covers out of these fabrics.  Wool can be upcycled.  Fleece can be found on sale.  Sorry no cheap pul/tpu options. 

Upcycled clothing and bedding towels. 
Go for it!  Just double check to know what it is for sure and check the fabric reference page for use.  T-shirts are best for decorative outers, towels tend to be Terry. And flannel can be found in bedding and flannel shirts.  Wool is often used for covers.  Make sure its wool of at least 80% wool and no cotton, hemp or bamboo-rayon as they will make it absorb. Minky cuddle blankets? Probably a  great decorative outer.  

Have fun thrift shopping, and for some fun, here's Macklemore for inspiration lol.