Cast Iron

1: Break off some of the little shiny specks.

2: See if those specks will stick to a strong magnet. (If not, that's a bad sign.)

3: Do one of those at-home test kits on only the specks.

If it didn't stick to the magnet and the test kit says positive for lead, then you know you're not getting a false positive on iron -- if it was iron or anything ferrous, it would have stuck to the magnet. That doesn't absolutely guarantee the accuracy of the test kit, but it would eliminate one common false positive.

If you can separate a sufficiently large speck, you could also do a hardness test on it: what materials is it able to scratch, and what materials rub material off of it? Lead is very much on the softer side, so if the specks are easily deformed and abraded -- especially when tested against things like aluminum -- that's another bad sign that it might be lead.

And, at any rate, I'd be leery of it if the specks don't stick to a magnet. The list of non-magnetic, silver-colored metals you want in contact with your food is pretty short, while the list of ones you don't want in your food is pretty long. Lead is probably the most likely and the worst offender on the list, but there are other possibilities that people might have been messing around with ... and most of those other possibilities aren't great to have in your food, either.