As Lewis suggested, hardness test it. If it’s close to pure lead, melt some of it down if there’s any “oatmeal” looking stuff on top as it just goes liquid, it’s probably zinc, and the stuff is garbage. You can get most of the zinc out with sulfur, but it’s dangerous and probably not worth your time.
You can experiment with alloying it with pure to get a specific hardness, and be able to use it up that way.
I have access to several tons of lead insulation bricks from an HDR vault at the hospital we are about to tear down. It has too much other stuff in it to make it worth my time. I was major bummed. I had hoped it would be pure lead....
If you don’t have a hardness tester, I have a video that shows a way to get a ball park on it. You need a scale, ball bearing, a way to push a couple hundred pounds of force, and calipers, and a known source of pure lead to check it against.