Secondary and tertiary RNA structures play an important role in many biological processes. Therefore the necessity arises to find similar higher-order structures for different but functionally homologous RNA sequences. We propose here a graph-topological approach to the problem, which shows two main features: simplified graph representation which allows the recognition of similarity of RNA secondary structures with the same branching look despite minor differences. This allows comparison among foldings from different sequences, and "pruning" of the secondary structures not shared by all the sequences since the early stages of the search. (b) The graph representation is encoded by the Randić topological index, and the search for the folding similarity is reduced to checking the identity of single numbers. These characteristics make this approach significantly different, less depending on empirical criteria, and less computationally heavy then previous methods, where the folding consensus has been measured by an alignment procedure or correlation of strings representing the secondary structures. Some U2 snRNA and viroid sequences are studied by this approach, which is imbedded in our previous search method based on genetic algorithms.