We are looking to distinguish between lead (II) fluoride (PbF2) and dinitrogen tetraoxide (N2O4). We really need to look no further than basic observation. Lead fluoride is a white, odorless, powdery solid. Dinitrogen tetraoxide can be either a gas or a liquid. N2O4 is in equilibrium with nitrogen dioxide, which has a brownish look to it. So no chemical reaction or experiment is required to distinguish between the two. If the unknown substance is a white solid, it's lead fluoride. If it's a colorless liquid or a gas, it's dinitrogen tetraoxide. Incidentally, you should be aware that both chemicals are quite toxic. Dinitrogen tetraoxide is a strong oxidizer and can be used as rocket fuel. The fumes are toxic to breathe.