My mum is an expert in finding fab things in charity shops, me - i'm set in my ways, I look at the books, look at the teacups, see if they've got any knitting needles (usually stashed in a cracked vase/pasta jar on the floor, behind the till), and then i'm done! Mum always finds great clothes - she can spot a 'label' a mile off, and then yesterday she found 5 skeins of Jaeger Natural Fleece yarn, just enough to knit a garment of some kind ...
She bought it because she saw 'Holmfirth' on the label - I lived there as a student, but it is lovely yarn. I don't tend to buy chunky in great quantities because I can't afford it. I usually treat myself to a ball of Rowan Big Knit every winter to make a beret!
I never spot any good yarn in charity shops, perhaps i'm not looking hard enough. I think with the advent of Ebay people tend to destash rather than donate, you may be lucky however, these are some tips for charity yarn buying ...
If the yarn still has a 'ball band' that's a good sign, at least you know the weight and the needles you'll need.
Check animal fibres for moth damage, you don't want to introduce the critters to your precious yarn stash! Look for tiny tiny white eggs in amongst the fibres.
Very often, charity finds are someone else's impulse buys, sets of skeins/balls which are together in a plastic wrapper are a good sign that the yarn will be from the same dye lot. If the skein/balls are loose in a basket or on a shelf, check that the dye lot is the same.
Random amounts of wool yarn are great for knitting up and felting.
Random amounts of bright acrylic yarn are just right for crochet granny squares, stripey crocheted ripple stitch blankets ... and yarn bombing projects.
Happy Hunting!!