Easy way to see how much of an allocated LUN is being used?

Without some kind of access to the host, a definitive amount could be difficult.  If it's a thick LUN, there really is no way that I'm aware of.  If it's a thin LUN, you can see how much space has been allocated vs the advertised size, but unless you are doing very regular block reclamations (which requires cooperation from the host), you may not know the real used number that the host would see.  So if you have a 5TB thin LUN, and the user write 1TB of space into it so ~1TB is allocated.  You can see that fact on the array.  But if the user then deleted 500GB from that 1TB of space, the array would not be able to determine that the space had been deleted (unless the host is doing some kind of block reclamation where it can tell the storage to free certain blocks.  Some hosts can do this, but it's not common everywhere and I, personally, don't see it much).

The fundamental problem is that the array typically doesn't know when a host frees up data in a LUN.  When a user or application deletes a file, most filesystems simply make metadata updates that mark those blocks free.  To the array, this just looks like a few writes as it has no insight into the host's filesystem so it cant distinguish between a data write and a metadata write.

It is a thick LUN and as I feared this was the answer.  Thank you very much.