How do you determine a hung mount? I don't mean if it's hung, I mean
which one is hung.

Here's the procedure so far:

run mount command
run df
look at last mount listed before df hung
force unmount the next entry in the mount command output

There's got to be a cleaner way than this, something that can be
scripted maybe?

Also, what determines if a mount is hung or stale? I can force a
mount to hang by just shutting off the nfs server service (we use
NetApp), but in the 5 cases of hung mounts in the last 4 years I would
say none were because nfs was off.