Apart from what's printed on the label, if you put your hand inside the sweater and can still see it, its single-ply. A two-ply fabric will feel bulkier and also yield more warmth. Although you may even find four-ply, six-ply, and eight-ply fabrics, a two-ply fabric is also considered good quality. But if you want more warmth, select from four-ply or higher.