At the moment, it is considered computationally difficult to determine whether or not two arbitrary graphs are isomorphic. You can read more about it on Wikipedia: Graph isomorphism problem - Wikipedia, the free encyclopedia
There are some necessary conditions that isomorphic graphs have: same number of vertices, edges, degree count, etc... If you can rule one of those out, then you can conclude that the graphs in question are not isomorphic. Otherwise, you will have to argue some other way (which can be difficult). Have you tried drawing the graphs and comparing them by sight?