Another approach is:

 select node_name from volumeusage where stgpool_name='________'

to verify if the nodes you expect are actually participating in a
group-collocated storage pool.

Notes:
- Nodes may participate in a group-collocated storage pool and
  not be defined in a collocation group, such that collocation by
  node would then be in effect for such clients, and make for more
  tape use than you might expect.
- The VOLUMEUSAGE table is expensive to search, so do only when
  necessary.

     Richard Sims    at Boston University