So your grandma has a bunch of fabric you can have since she won't use it anymore? Or there's a bag full of spools of thread for a buck at that garage sale? A couple yards of something hanging there at your local goodwill for dirt cheap?
Well, you're probably not going to be at a great loss at thrift store/relative give-away prices but Sew-er Beware! It only takes a couple years of sitting on a shelf in the basement or in the attic or even in a sewing room for dry rot to set in. You may not even notice the problem until your garment is completed and seams practically melt apart from thread that's disintegrated or fabric in high stress areas just pulls apart like a really tender slow-cooked pot roast.
My grandmother's house was being cleaned out and consolidated when she moved into an assisted living home and I 'inherited' a huge ziplock of thread and some other fabrics that had been sitting around unused in her upstairs un-air-conditioned sewing room in Pennsylvania. The threads I all tested and only a few were sound enough to use. 90% of that huge bag went right into the trash. The fabric appeared okay and I didn't think to test it further. Unfortunately what seemed like a great chunk of black cotton bottomweight turned out to be on the verge of rot and after being used as a doublet lining for a summer at Bristol it completely gave out at seems, under arms, around grommets, etc etc etc. Now, because I forgot where this fabric came from and didn't bother to do some basic tug tests on it, I'm replacing the entire lining for this garment. When I pulled out the existing lining made from said fabric it looked like I was holding an extant piece from the period. All those pictures in Janet Arnold's Patterns of Fashion? Yeah, it looks just like that.
So learn my lesson the easy way. When you acquire fabric and thread from unknown storage backgrounds make sure the integrity is sound BEFORE you use in your projects. Give thread a firm tug, try breaking the thread from a spool you just bought to compare. Does it snap after a hard pull that probably leaves lines on your hand? Or does it pop apart easily, pulling apart with a moderate or light tug? Cut a medium swatch of the fabric and try pulling it apart (not trying to rip it with a started notch). When it rips what do the threads look like? Rotted thread will be very loose, fuzzy, or frayed and the fabric will tear with very little force.
Prevent your own stash from suffering this fate by making sure fabrics are stored in cool dry places, sealed in containers or zip locks if possible. Use dehumidifiers in basements and store fabrics in an air-conditioned area if you live in a humid environment.