Exothermic or Endothermic reactions

Can we determine whether a reaction is endothermic or exothermic? For example if we are given the following reactions:

$$\ce{2SO2 + O2 -> 2SO3}$$

$$\ce{H2 + I2 ->2HI}$$

$$\ce{N2 + 3H2 -> 2NH3}$$

Can we determine weather these reactions are endothermic or exothermic or do we have to just memorize this? Are there any general trends that could make memorizing this information more manageable?