All intermediate states in a reaction are known as stationary points, for example, reactants, transition states, intermediates, products. One way to find the difference between them is by looking at the frequency of their normal modes. In order to verify if a stationary point is a transition state, you have to do a vibrational frequency calculation at the same computational level as we do the geometry optimization. The normal mode corresponding to the imaginary frequency in the transition state usually reflects the change in geometry in going from reactants to products.