Highlight mixed and trailing whitespace

.vimrc

" Bad whitespace shows up in red.
highlight BadWhitespace ctermbg=1 guibg=Red
" This function will clear bad whitespace highlighting in help files.
" It is needed because Filetype events fire after BufEnter ones. See below.
function! IgnoreHelpFiles()
    if &filetype ==? 'help'
        call clearmatches()
    endif
endfunction
" Match whenever we enter a buffer. (These happen in order)
augroup badwhitespace
    autocmd!
    " Tabs at the beginning of a line are bad. Especially if they are mixed in with spaces.
    autocmd BufEnter * call matchadd('BadWhitespace', '^\( *\t\+\)\+')
    " Trailing whitespace is bad, too.
    autocmd BufEnter * call matchadd('BadWhitespace', '\s\+$')
    " Remove matches in help files.
    autocmd BufEnter * call IgnoreHelpFiles()
augroup END

Add this to your .vimrc to highlight spaces and tabs at the end of lines, and tabs at the beginning of lines.