I generally just try to get a 'ballpark' idea of what an alloy is by:
1. Smacking it with, or dropping it on something hard, and listening to the sound. The more pronounced the ring, the harder it is (generally).
2. Inspecting the grain structure. There's no real science here, just me making a guess about the alloy, based on the different amount of visible dendritic formation from alloy ingredients like antimony and tin. And, luster helps. Linotype makes beautiful, shiny ingots. Lead and lead-tin alloys ... not so much.
3. Grab some known alloys and smack the ingots together. I have pure lead, pure tin, isotope core alloy, COWW, and linotype to compare with. In a smash battle, softer ingots take more damage.
If that doesn't provide enough information for a reasonable guess, I might cast a few bullets and see how they compare to known alloys.
And if that still isn't enough, or I don't feel like casting samples, I just chuck it in the 'scrap' pile that gets combined with range lead.
One of these days, I might try the 'artists' pencil trick (or is it architects' pencils?). But, until then, it's mostly caveman-style.