Frequency calculation can be done in Gaussian using the following steps:

1. Once you have obtained the optimized structure for all reactants, products and transition states, save their coordinates. Now you have the coordinates of stationary points.
2. Using the new coordinates perform frequency calculation at the required level of theory.
3. After the calculation is done, open the vibrations section from Results. The table contains all frequencies of the vibrational modes.
4. Compare the normal modes for reactant and products with that of the transition state.
5. Minimum energy stationary points will have all positive valued frequencies while a maximum energy stationary point will have one negative frequency.
6. The motion of the atoms in the negative frequency vibrational mode should be along the reaction coordinate which connects the reactants and products,
7. In case, the negative frequency corresponds to a different mode, the stationary point does not connect the desired reactants and products and it might be a transition state of a different reaction.