So you compare the enthalpy of the reactants with the enthalpy of the product(s), then you can figure out endo or exo, right? Sounds easy enough. If the product is solid and the reactants gas, that means the kinetic energy of the atoms goes way down, so that would be a pretty good cue its endothermic. If the volume greatly increases, maybe it melted or sublimed, and therefore likely exo. For calculations, assume pressure is at 1 atm?