if you have the original package or remember what the package was then fine but if it is unknown I wouldn't trust my gut/ way too many bad things lead does to living things to make me comfortable to leave in lakes and rivers let alone aquaria.
lead is extremely soft metal- try to scratch it or drop it on a hard surface - should leave a nice scratch or flat mark if its lead- as tungsten and alternative to lead weights are usually more expensive and harder to find i'm pretty sure you have lead weights.