To tell if something is handmade or mass-produced I would look at the seams. Mass-produced sweaters and things tend to be cut out of large pieces of flat machine knitting and then sewn together with a sewing machine. Hand-seaming will look different. Or you could look at the size of the yarn. Look at a T-shirt, or underwear, or a commercial sock. No one could knit with yarn that small, you'd need like 000000000 needles.