To really tell via the "scratch test" requires that you take a rat tail file to one of the holes for a lug and ream it out a bit. If the resulting shavings are silver then it's aluminum, if they are a gold tone then it's brass (or possibly bronze). A bit extreme but it does answer the question once and for all.
I've done the scratch test on 5 or 6 Supras over the years. As Kev said, you have to file a bit more than 'a little' as you might just reach the copper plate and conclude it's brass. If the shavings start to add up as a gold color, it's not aluminum Ludaloy, but a copper containing alloy-either brass or bronze.
There was a guy on DFO that had an early 60s steel (or at least a pretty significant amount of 'ferrous-ness') Supra.