This is a bit OT but hopefully informative. The answers that stainless steel spokes are nonmagnetic are correct because the stainless used in spoke manufacture (most commonly AISI Grade 304) is an austenitic stainless and, therefore nonmagnetic. So while a magnet is a definitive test for stainless steel spokes it also won't attract the far less common aluminum spokes (trade named Zircal) Mavic used on some of it's boutique wheels and the also rare and very pricey Titanium spokes that were trendy a few years ago.

Further OT, not all stainless steels are nonmagnetic. The 300-series (303, 304, 316, etc) used in spokes and for many industrial products for it's high corrosion resistance is nonmagnetic. However, the 400-series (410, 416, 440, etc., aka stainless "tool steels") used for cutlery, tools and similar products are fully magnetic and cannot be told from regular carbon steel by using a magnet.