I have a small foundry, and I have been able to get all my metals for it, from my scrapping efforts. Die casting alloys are based around Zinc (Zn), and are great because of the low melting temps and its ability to transfer high detail from the molds. Zinc alloys are best used in parts that dont have any real structural duties, because of its tendency to fracture easily under stress loading (Most Zinc alloys contain Aluminium in them). I need to be careful that I always keep Zinc out of my Aluminium alloys when I need to make any structural items, because of its ability to fail just when you need it most.
Die cast alloys are easy to spot from the surface texture of the part you are looking at. If its all smooth and has an almost silky feel to the part, with high detail and crisp edges, then it is most likely to be die cast. Cast Aluminium parts are generally done using sand casting methods and tend to have a grainy surface to them, with more rounded edges and less detail. Most Aluminium car engine parts (like intake manifolds and coolant fixtures, bell housings, etc) have the telltale signs of being sand cast on their surface and design.
The acid test mentioned above, refers to the presence of Magnesium (Mg) in the alloy, which can be a real hazard to the hobbyist like myself (auto ignites in the furnace and forms thermite that burns at over 3000 degrees). I test every single piece of alloy scrap now for Mg, by filling a small area on the scrap and putting a drop of white vinegar on it. If the freshly filled sample starts to fizz or turn black, then it has Mg in it, and it goes into a holding bin, well away from my other alloys. On guy falkes night, I always have a small film canister with some fillings from some Mg scrap, thrown on an outdoor fire from a safe distance, it makes a great display.
If you do get enough Magnesium alloy gathered up, it can get a much better price than standard cast Al alloys, as its used in high end castings due to its strength and light weight compared to all other alloys (and it needs a special furnace with an inert gas atmosphere).
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