First Make Adjacency matrix for both graphs.
These matrices would be square and symmetrical. (If No multiple or directed edges)
The main diagonal would be all zeroes (if no loops)
if the number of 1s and 0s are not the same in both matrices then it's not isomorphic surely. If they are the same then it may be isomorphic.
How do we prove that it may be or may not be isomorphic. Can you give some reason?