My vim doesn't use iwhiteeol by default:

                        *'dip'* *'diffopt'*
'diffopt' 'dip'     string  (default "internal,filler")
            global
            {not available when compiled without the |+diff|
            feature}
    Option settings for diff mode.  It can consist of the following items.
    All are optional.  Items must be separated by a comma.

But to disable it:

set diffopt-=iwhiteeol

My vim doesn't use iwhiteeol by default:

                        *'dip'* *'diffopt'*
'diffopt' 'dip'     string  (default "internal,filler")
            global
            {not available when compiled without the |+diff|
            feature}
    Option settings for diff mode.  It can consist of the following items.
    All are optional.  Items must be separated by a comma.

But to disable it:

set diffopt-=iwhiteeol

You can also

set list listchars^=trail:.

to show trailing whitespace