There are a couple of ways to determine if a reaction is exothermic or endothermic. Endothermic meaning that heat is added to the reaction to make the reactants interact and exothermic meaning heat is released during the reaction between the two reactants.
In a chemical equation the word heat or the triange symbol are used to indicate if the reaction is exothermic or endothermic. Thus, Na + HOH --> NaOH + heat would indicate the reaction is exothermic. On the other hand, an equation such as 2H2 + O2 +heat --> 2 HOH would indicate that the reaction is endothermic and no reaction will take place unless heat is added.
Another way is to look at the energy diagram for the reaction. In an energy diagram the energy is on the y-axis and time is on the x-axis. The reactants are shown first at their energy level, and the products at their respective energy levels. In the middle between the two is the activation energy required to transform the reactants into products. If the energy level of the reactants is higher than the energy level of the products the reaction is exothermic (energy has been released during the reaction). If the energy level of the products is higher than the energy level of the reactants it is an endothermic reaction.
If the energy is not shown in the chemical reaction and there is no energy diagram to look at, a last alternative is to calculate the total bond energy of all the chemical bonds in the reactants and in the products. There are standard bond energies between almost all elements and you just have to look them up and do the math.
Endothermic reactions need activation energy to proceed. Therefore, heat energy is needed for an endothermic reaction to occur. The prefix endo- means within, therefore energy must be absorbed for this type of reaction. Usually in this type of reaction, heat energy that is absorbed is changed into chemical energy. An example of an endothermic reaction is the melting of ice. Energy is required for this to occur. With chemical reactions, the added heat energy would be seen on the left side of the equation where the reactants are.
An exothermic reaction releases heat energy. The prefix ex- refers to outside and heat energy is released at the end of this type of reaction. Examples of exothermic reactions are when acids and bases are combined in a neutralization reaction--the products formed are salt and water and heat energy gets released as well. Combustion of fuels is an exothermic reaction. When fuels combust, heat energy is released. An example of this is when fossil fuels are burned in engines of cars, or in oil or gas heaters.