Not that I could reliably figure out.  Not without going into the internals of the ZMQ types, which is kind of a no-no (see: http://api.zeromq.org/3-2:zmq-msg-data -- I mean, the signals always have messages that are 24 bytes...  But then, so do strings sometimes.  But as far as I can see, one thing that ZMQ3 doesn't actually have a function for determining what's in a message).

This is my least favorite bit.