Energy is conserved in reactions, meaning it is never created or destroyed but rather transferred to or from a reaction environment. Energy is gained and lost from a reaction mixture as bonds are broken or made.
If a reaction gives out heat it is exothermic and if it draws in heat it is endothermic.
Experimentally you can determine if a reaction is endothermic or exothermic by checking for energy given out or taken in. Simply this can be done by feeling the reaction container or using a thermometer for heat change, and observing any light energy given out.