Is it possible for nodes to 'detect' which networks are available ?
Part of the configuration of an XBee is THE network to connect to. The XBee can then determine if THAT network is available.
If you mean something else, like the XBee should be able to change it's PAN ID, then, yes, it is possible for the Arduino that the XBee is connected to to re-configure the XBee to use a different PAN ID, and then the XBee can try to connect. If that fails, try the next PAN ID, until a connect happens.
That is NOT a trivial process, but it can be done.