Hi,
Two plugins may be useful here:
1) Graph Merge: Use Intersect and Difference options to find common nodes and edges. The plugin is available from within Cytoscape: Plugins -> Merge networks. Please note that the plugin considers edge directionality when searching for similar edges.
2) Network Analyzer. Compute specific parameters describing network topology, and compare these params. This plugin is available here:
http://med.bioinf.mpi-inf.mpg.de/netanalyzer/
Hope this helps,
Maital