I have a 14x5.75 set, they fizzle nicely under vinegar. Definitely a mag+alu alloy.
a pretty darn easy way to guesstimate the amount of mg in the wheel is how dark untreated areas get. if its has high mg content it becomes dark grey from exposure like the sexy gta/gtam autodelta mg valve covers and bellhousing.
you can see how dark they get...