Be advised that you can place a magnet directly on an actual aluminum rim only to potentially discover that the rim appears to be attracted by the magnet! Thus, this would seem to suggest that the rim is made of steel when in fact the rim could be made of aluminum with steel joining pins at the rim’s joining seams.
Hence, apply the magnet at various points along the rim to determine the rim’s aluminum or steel composition or you might just coincidentally apply the magnet at its joining seams and detect the steel joining pins and be given a false impression of the rim’s composition. The Bontrager rims on my road bike give this false impression at their seams only (which is precisely where I coincidentally initially tried the magnet trick).