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.