There seems to be several ways to handle this.

Modelines

gedit has a modeline plugin. If you enable it you can use the Emacs modeline option Indent-tabs-mode (or any other supported modeline option with the same effect). By setting that option to true you can make gedit indent with tabs for the file in question. So, to enable tab indentation in a Makefile add the following line to it:

# -*- indent-tabs-mode:t; -*-

Makefiletab

There is a gedit plugin named Makefiletab which is said to "force the option spaces-instead-of-tabs off for all Makefiles." I do not know if it works though as I haven not tried it.

As the file is UTF-8 you could run isutf8. An additional utils package. It gives you both line, char and offset for bad bytes.

Then use xxd, hexdump or the like to analyze.

Unfortunately it stops at first crash. But then again it depends on the file. Could be there is only one bad byte ;)

_{Have some C code that does a similar analysis but for entire file. It is on a disk somewhere long forgotten. Could try to find it if in need.}

Else yes, the quick and not that dirty way would be to do a diff between a copy saved with gedit – as proposed by the good mr. @vonbrand.