You must performed: (a) localisation of TS structures using (for example qST2 or qST3 methods), (b) vibrational analysis, (c) IRC calculations - these calculation should be connected transition structures with suitable minima of products an substrates (or pre-reaction complexes - pre-reactions complexes are formed from substrates without activation barrier). In the case of two-step cycloaddition additional calculations are necessary - localisation of second transition state and finding of connection with zwitterionic/biradical intermediate. For detail please write to my email: [email protected]

Thanks but is it possible to follow the same with PES (Potential Energy Surface) scan?

generally - yes :)

Technically, mapping the ENTIRE reaction pathway is impossible because the starting point is one in which the reactants are infinitely separated.  Practically, one only needs to follow the reaction path far enough in either direction from the TS to confirm that the TS does indeed connect the reactant and the product.  An IRC calculation is the best way to do this.  You could do a relaxed PES scan (as Jasinksi says) but, except in the simplest cases, it would be difficult to chose the right set of geometric parameters to use as the independent variable. 

Thank you.