As Steven indicates, there is no ready way to determine which volumes a
restore will use.  (A "-preview" client option is very much needed.)
But as with all things, there are always crude "trick" methods to
accomplish an end...

Volumes for restoral, determine         How to determine which tapes will be
                                        needed in a restoral?  One crude but
                                        effective method is to change all that
                                        node's volumes to Unavailable status,
                                        run the restoral on the client (which
                                        will obviously be unable to proceed),
                                        and then view the error messages in the
                                        server Activity Log, which will identify
                                        the tapes.

 Richard Sims, BU