$ grep '<ctrl-v><tab>' your_file
remember, vim would expand the tab to spaces we input, so the grep will fail us.
$ grep expandtab ~/.vimrc
if the character is a tab, the cursor would stop once;
if they are spaces, the cursor will stop for a "tabstop" times.
$ grep tabstop .vimrc
with hexedit, or hexdump, or any hex editor,
we can see the TAB is ascii 09,
while we know space is ascii 20.