How do you know if a reaction is endothermic?

If the products are at a higher energy level than the reactants, the reaction must have absorbed energy. If you have to heat the reactants to keep the reaction going or if it cools down during the process, the reaction is endothermic.

If you have to heat the reactants to keep the reaction going or if it cools down during the process, the reaction is endothermic

the electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, x-rays, and gamma-rays.

thinking it would be physical characteristics