Aluminum vs stainless steel
Both are silver and neither sticks to a magnet, so the magnet won't help you here. Pick it up instead.
Lift it
Stainless steel weighs about three times as much as aluminum the same size. A stainless sink feels solid; an aluminum one feels like it could blow away. This is the fastest test.
Scratch it
Aluminum is soft. A key or file scratches it easily and leaves a dull white mark. Stainless is hard and stays bright; a file barely bites.
The magnet, carefully
Most stainless (304 and 316) doesn't stick, or pulls weakly where it was bent. Cheaper 400-series stainless does stick, and pays less. So a magnet tells you about stainless grades. It won't tell aluminum from 304.
Watch for chrome
Chrome-plated steel looks like stainless but grabs a magnet hard. File an edge and you'll see grey steel under a thin shiny skin. It sells as steel.