How do I receive an alert message indicating that persistent undo will be ignored?

Question

I am using Vim 7.4. According to vim's help on persistent undo:

An undo file is also ignored if its owner differs from the owner of the edited file. 
Set 'verbose' to get a message about that when opening a file.

Where do I set verbose? In my vimrc I have the following as my persistent undo-related code:

if has("persistent_undo")
  if isdirectory(expand("~/.vim/temp_dirs/undodir"))
    verbose set undodir=~/.vim/temp_dirs/undodir
  else
    echomsg "ERROR: undodir does not exist or is not writable."
  endif

  verbose set undofile
endif

However, when I edit a file of which I am not the owner, I get no such message.

Answer 1

The sentence

Set 'verbose' to get a message about that when opening a file.

Refers to the option verbose—you can tell because it's enclosed in single-quotes, and if you place your cursor on it and press Ctrl-], you jump to the same section as :help 'verbose' would.

It's not clear what level of verbosity you would need, but try

set verbose=1

and then increase from there.